Spur Gear
"Spur gears are the most easily visualized common gears that transmit motion between two parallel shafts. Because of their shape, they are classified as a type of cylindrical gears. Since the tooth surfaces of the gears are parallel to the axes of the mounted shafts, there is no thrust force generated in the axial direction. Also, because of the ease of production, these gears can be made to a high degree of precision. On the other hand, spur gears have a disadvantage in that they easily make noise. Generally speaking, when two spur gears are in mesh, the gear with more teeth is called the “gear” and the one with the smaller number of teeth is called the “pinion”.
The unit to indicate the sizes of spur gears is commonly stated, as specified by ISO, to be “module”. In recent years, it is usual to set the pressure angle to 20 degrees. In commercial machinery, it is most common to use a portion of an involute curve as the tooth profile.
Even though not limited to spur gears, profile shifted gears are used when it is necessary to adjust the center distance slightly or to strengthen the gear teeth. They are produced by adjusting the distance between the gear cutting tool called the hobbing tool and the gear in the production stage. When the shift is positive, the bending strength of the gear increases, while a negative shift slightly reduces the center distance. The backlash is the play between the teeth when two gears are meshed and is needed for the smooth rotation of gears. When the backlash is too large, it leads to increased vibration and noise while the backlash that is too small leads to tooth failure due to the lack of lubrication.
All KHK spur gears have an involute tooth shape. In other words, they are involute gears using part of the involute curve as their tooth forms. Looking generally, the involute shape is the most wide-spread gear tooth form due to, among other reasons, the ability to absorb small center distance errors, easily made production tools simplify manufacturing, thick roots of the teeth make it strong, etc. Tooth shape is often described as a specification in drawing of a spur gear as indicated by the height of teeth. In addition to standard full depth teeth, extended addendum and stub tooth profiles exist. General Tooth Form
The tooth form of spur gears is normally shown as a plane curve on the cross section perpendicular to the shaft. Therefore, instead of pitch cylinder, pitch circle is used. The contact point of the two pitch circles is called the pitch point. The pitch point is the point that the two pitch circles touch in rolling contact so that it is the spot that has no relative motion between the gears or, in other words, the instantaneous center of relative motion.
In Figure 2.1 where the pitch point is P, the contact point of the two gears is C, consider the common normal of the two tooth forms CN and the common tangent CT.
Necessary Conditions Of Tooth Form Mechanics
Figure 2-1 Necessary Conditions Of Tooth Form Mechanics
In order for the two tooth forms not to separate or to run into each other as they rotate, there can be no relative motion in the direction of the common normal. That is, the velocity component must be equal. However, there is no problem if there is relative motion in the direction of the common tangent. That is, there can be a different velocity component, which is the sliding between the tooth forms and the difference of the velocity components, vs, is the relative sliding speed. Therefore, the relative motion at the contact point C is limited to the direction of the common tangent CT. However, as well-known in kinematics, the instantaneous center of relative motion is in the straight line perpendicular to the direction of the relative motion. Therefore, ”The common normal CN at the contact point C of the two tooth forms must go through the contact point of the pitch circles, that is, the pitch point P (instantaneous center).”
This is called the necessary conditions of tooth form mechanics which was attributed to Camus in 1766 in France and forms the foundation of tooth form theory. As long as a curve satisfies this condition, and the two tooth bodies do not interfere with each other, it can be used as a tooth form.
Cycloidal Curve
Figure 2.2 Cycloidal Curve
For example, as shown in Figure 2.2, when the pitch circles O1 and O2 are externally touching, and when the circle Or2 , which is internally contacting O2, rolls while contacting the two pitch circles at pitch point P, consider the traces F1 and F2 that a point C draw on the gears. The pitch point P is the instantaneous center of relative motion of the two pitch circles and circle Or2 (which is called rolling circle), and the straight line CP becomes the common normal of the traces F1 and F2. Therefore, F1 and F2 satisfy the necessary conditions of the tooth form mechanics. As a result, F1 can be used as the tooth form outside of the pitch circle of gear 1 and similarly F2 for gear 2. It should be noted that when the radius of the rolling circle Or2 is less than the radius of pitch circle O2 , there is no interference. The curve F1 is called epicycloid while F2 is called hypocycloid. This kind of gear is called cycloidal gear. Similarly, think of rolling circle Or2 internally contacting pitch circle O1, then we can obtain the tooth form inside pitch circle of gear 1, F1’ (hypocycloid) and tooth form inside pitch circle of gear 2, F2’ (epicycloid).
In general, using the pitch circle as the border, the tooth surface close to the toe of the tooth is called tooth face, and the tooth surface close to the heel, tooth flank.
Cycloidal Gear
Figure 2.3 Cycloidal Gear
The externally contacting cycloidal gear’s tooth face is epicycloids, and the tooth flank is cycloid. As the circle Or1in Figure 2.3, when the rolling circle’s diameter is equal to its internally contacting pitch circle’s radius, the hypocycloid that results becomes the diagonal straight line; when less than the radius, concave in looking from the pitch point and convex when greater than the radius. The tooth form curve of a rack paired with a cycloidal gear is when the pitch circle becomes a straight line and is called the common cycloid.
The trace point C may not always be on the rolling circle. When the point C is inside or outside of the rolling circle, the tooth form is called trochoid.
In case of a cycloidal gear tooth form or a trochoidal tooth form, when the rolling circle and its internally contacting circle is matching, point C becomes the one side’s (equivalent to hypocycloid) tooth form. This is called point tooth form. Next, if the circular arc with point C as its center (concave or convex) is used as the tooth form, the mating tooth form becomes the curve equidistant at radius equal to the arc with cycloidal or trochoidal arc of point C.
Figure 2.4 shows one kind of such gears when one side of the pair uses pins and called a pin gear.
One Kind Of Pin Gear
Figure 2.4 One Kind Of Pin Gear
By generalizing the cycloidal and trochoidal gear forms, instead of a rolling circle, using any curve with its curvature greater than the interior contacting circle, regardless of the pitch circle, mutually meshing same system tooth forms can be obtained as the trace of the common contact point.
Also, instead of the pitch circle, by considering arbitrary curves which are in mutually rolling contact as the pitch curves, their trace points lead to tooth forms of non-circular gears.
The aforementioned cycloidal and pin gears were often used in the early stages of gear development, but now they are very limited in their use such as in clocks, and general machine industries mainly use involute gears which are discussed next.
Involute Tooth Form
An involute curve is produced by the trace of any point on a non-stretchable string which is wound on a circle as it is unwound under tension. It is the involute of a circle. The following is the explanation of why this curve is suitable as a tooth form.
Involute Tooth Form
Figure 2.5 Involute Tooth Form
As shown in Figure 2.5, if we cross wind a belt around circular belt wheel B1 (diameter dg1 , angular velocity ω1 ) and B2 (diameter dg2, angular velocity ω2 ) and rotate it under tension in the direction of arrows, point C (CI or CII ) on the belt whose traces of the points F1 and F2 are clearly involute curves. If these are considered to be the tooth forms and if the points I1 and I2 are the points where the belt leaves the belt circles, then the straight lines CI1 and CI2 are, by the nature of involute curve, normal to respective tooth forms F1 and F2. If the intersection of the line I1 I2 and the center line O1 O2 is point P,
O1P/O2P = dg1 /dg2 = ω1 / ω2
Therefore, O1P • ω1 = O2P • ω2
and point P is the pitch point. Thus, the necessary conditions of the tooth form mechanics are satisfied and point C is the contact point with its trace becoming a part of the straight line I1PI2. Also, points I1 and I2, due to the nature of involute curve, are respectively the centers of curvature of the tooth forms F1 and F2, thus there will be no interference at contact points. The contours of these belt wheels, circles B1 and B2 , are called base circles, the common tangent to the base circles, I1PI2, is the line of action, and the angle formed by the line of action and the common tangent of the pitch circles, PX, at the pitch point is called the pressure angle. If we assume the center distance O1O2 is A and d1 and d2 to be the diameters of the pitch circles, then,
cos = (dg1 + dg2)/2A, d1 = dg1 sec , d2 = dg2 sec
The values of pressure angle most commonly used are 20° and 14° 30 ‘.
The line of action indicates the direction of the force acting normal to the tooth surface. Ignoring friction force, if the transmitted torque of i wheel is Ti and the radius of pitch circle is Ri, then, the normal tooth surface load F is expressed as:
F = Ti / Ri cos α
At the same time, this becomes the radial load on the axis. Therefore, with the same transmitted torque, the larger the pressure angle α, the greater the normal force on the tooth surface and consequently on the shaft.
The distance measured on the pitch circle between the adjoining teeth, p, is called the circular pitch; and the distance, t, measured between the adjoining teeth on the base circle which is the same as the distance on the common normal on the same side of the adjoining tooth form, is called the base circle pitch, base pitch or normal pitch. If the number of teeth is zi (i = 1, 2), the relation is (See Figure 2.6):
p = π di / zi , t = π dgi / zi = p cos α
Normal Pitch
Figure 2.6 Normal Pitch
(Pitch circle, normal pitch, base circle pitch, base circle : From left to right)
When we simply speak of pitch, we mean circular pitch and the larger the circular pitch, the bigger the gear teeth. Therefore, we could use it to indicate the basic sizes. However, as shown above, it includes the irrational number, π, which makes it inconvenient. For that reason, the number obtained by the pitch diameter in mm, di, divided by the number of teeth, zi, is used to express the basic sizes. This is called module and is indicated with the symbol, m. Thus,
m = di / zi
and p = π m , t = π m cos α
However, this is in the metric system which is used in Japan, and various European countries, but in England and the US where the inch system is used, the value obtained by number of teeth, zi, divided by the pitch diameter in inches, di”, is used. This is called the diametral pitch, D.P., and shown with the symbol, P. This is similar to the expression in screws of indicating by so many peaks per 1”, that is:
P = zi / di” , di” = di / 25.4
P = 25.4 / m , that is mP = 25.4
Table 2.1 shows the standard module specified by JIS B 1701 and the standard D.P. along with their equivalent modules. (Additional data shown at the end.)
Circular pitch, module and diametral pitch are used in other kind of gears, but the normal pitch is unique to involute gears.
In Figure 2.5, if the tangent to the pitch circle PX at pitch point P is the pitch line of a rack, and the intersection of tangent to pinion tooth at point CI, F∞ (orthogonal to line of action) and PX as point P∞, then P P∞ = PCI sec α and F∞ moves parallel to at the same speed as [the product of sec α and the belt speed rgi ωi ] = ri ωi = [speed of the rack]. Therefore, the straight line F∞ matches the tooth form of the rack. The fact that the tooth form of a rack mating with an involute gear is a simple straight line makes its characteristics very convenient for gear cutting and inspection.
The involute curve itself is determined by the size of the base circle and since one tooth surface from the tooth flank to the tooth face is made up of one continuous curve, it is called a single curve system and is differentiated from the one made up of different curves for tooth face and tooth flank, such as a cycloidalal tooth form, which is called the double curve system.
Table-2.1
(module, diametral pitch, inch module : From left to right)
Involute tooth form is a single curve system and the angular speed ratio is inversely proportional to the radii of the base circles. While a change in the center distance produces a change in the pressure angle, it does not affect the angular speed ratio. Therefore, even if there is an error in the center distance at assembly, the correct angular speed ratio is maintained. By taking advantage of these characteristics, it becomes possible to make improved gears called profile shifted gears which have a significant practical importance. When meshing a rack and a pinion, the rack for a particular involute pinion can have the pressure angle and pitch be varied depending on how the pitch circle is defined (p cos α = t = const.).
For external meshing, the involute gears always mesh on convex surfaces. On the other hand, for cycloidal gears, when the rolling circle diameter is less than the radius of its internally contacting pitch circle, a convex and concave surfaces will contact giving advantage to the mesh as well as giving stable sliding rate (later discussed) resulting in almost equal friction on tooth surfaces. These and other theoretical benefits as well as the possibility of gears with smaller number of teeth give cycloidal gears design advantages and was being recommended. But they were insufficient to overcome practical advantages of involute gears as above mentioned, and the cycloidal gears ceded the top spot to involute gears. There are various ways to express involute tooth forms but as shown in Figure 2.7, if we express the tangent angle, τ * , as the intermediary coefficient with polar coordinates ρi , φi , then,
Arc Ii C0 = Ii C = (dgi /2 ) tan τ
φi = ∠ Ii OiCo – τ , Arc Ii Co = (dgi / 2 ) • ∠ Ii Oi Co
Involute Function
Figure 2.7 Involute Function
* Tangent angle τ is also called in broad sense the angle of obliquity of action.
Therefore, you obtain: ρi = (dgi / 2) sec τ , φi = tan τ – τ
This tan τ – τ is called involute function and is written as inv τ. That is φi = inv τ.
If we eliminate τ from above equations:
φi = [√ (2 ρi / dgi )2 – 1] – cos -1(dgi / 2 ρi )
Also, CIi [ = (dgi / 2) tan τ ] is equal to the radius of curvature of the involute tooth form at point C. Thus, Ii is the center of curvature.
Table2.2 Recommended Values of Involute Full Depth Tooth Basic Rack (JIS)
Pressure Angle α : 20°
Tip Clearance c : 0.25m
Tooth Bottom Fillet γ : 0.375m = 1 1/2 c
Applicable To : Cut tooth form
Pressure Angle α : 14.5°
Tip Clearance c : 0.25m
Tooth Bottom Fillet γ : 0.333m = 1 1/3 c
Applicable To : Cut tooth form
Pressure Angle α : 20°
Tip Clearance c : 0.35m
Tooth Bottom Fillet γ : 0.300m
Applicable To : Ground and shaping tooth form
Pressure Angle α : 14.5°
Tip Clearance c : 0.35m
Tooth Bottom Fillet γ : 0.300m
Applicable To : Ground and shaping tooth form
Note: When using other than the values c given above, make c ≥ 0.157 m (= p /0).
Basic Gears
The right and left surfaces of spur gear tooth are normally made symmetrical and they are considered to be basic when the tooth thickness measured along the pitch circle is ½ of the circular pitch. This is called basic gear. And the rack obtained by increasing the pitch circle to infinity is called basic rack.
Basic Rack
Figure 2.8 Basic Rack
(meshing gear’s pitch circle, meshing gear’s basic rack, meshing gear, basic rack, root line, tooth tip line, pitch line (for tooth cutting) : From left to right)
Figure 2.8 shows the basic gear and the basic rack of the full depth tooth involute spur gear and Figure 2.9 shows the basic rack according to JIS B 1 701 and its recommended values in Table 2.2.
20° Full Depth Tooth
20° Full Depth Tooth
14.5° Full Depth Tooth
14.5° Full Depth Tooth
Figure 2.9 Basic Racks of JIS
Here we define full depth tooth as one in which the height of the tooth from the pitch circle to the tooth tip, that is its addendum, is equal to its module. It is equal to the addition of the height from the root to the pitch circle, that is its dedendum, and the clearance, c (= km), space between the tooth tip and the root of the opposing gear. Therefore, the whole depth of a full depth tooth is; 2m + c.
In actuality, in order to avoid meshing interference due to errors in pitch or tooth form or from the dimensional deviation due to thermal expansion, a gap is given on the backside of the teeth. This is called backlash.
The basic gear from which the tooth thickness is reduced by the necessary amount of backlash is called standard gear and is shown in Figure 2.10. In our country, the pressure angle of 20° (when large tooth strength is desired) or 14.5° (when large combination of teeth, that is high meshing ratio, and low bearing load are desired) is used. But in order to further increase tooth strength, 22.5°, 25°, and 26.5° are sometime used. In Germany, England and Russia, only 20° is standardized (see reference at the end).
Nomenclatures Of Parts Of Standard Spur Gear
Figure 2.10 Nomenclatures Of Parts Of Standard Spur Gear
Larger gear
Number of teeth of larger gear, z2
Line of centers
Smaller gear
Number of teeth of smaller gear, z1
Center distance a = (z1 + z2) m / 2
Effective tooth height
Addendum
Whole depth H = (2 + k)m
Dedendum
-
Point of interference
Root
Working depth clearance = km
Pitch point
Line of action
Normal pitch
Base circle pitch = normal pitch t
Root diameter = (z1 – 2 – 2k) m
Base circle diameter dg1 = z1 m cos α
Base pitch diameter = d1 = z1 m
Outside diameter = dk1 = (z1 + 2) m
Root fillet
Contact length S
Path of approach S1
Path of recess S2
Two pairs meshing
One pair meshing
Backlash ƒn
The height of tooth most often used is the previously mentioned full depth tooth. But when an especially strong tooth is needed, stub tooth gear with 20° pressure angle and addendum of 0.8m is used. Also, when the pressure angle is relatively large (for example, 20°) and an increase is wanted in the contact ratio, a “high” depth tooth (addendum > m) is sometimes used. The backlash, ƒn, measured in the direction of the line of action (normal to the tooth surface) is usually more or less about 0.04 m. The backlash is sometimes expressed on the pitch circle, but it is more common recently to indicate the value on the line of action. The value on the pitch circle is the value on the line of action multiplied by sec α.
The following formulas give the various dimensions of the externally meshing standard full depth gears. Here z1 and z2 are the numbers of teeth of the respective gears.
Center distance
a = (z1 + z2) m / 2
Base pitch circle diameters
d1 = z1 m , d2 = z2 m
Outside diameters
dk1 = (z1 + 2) m, dk2 = (z2 + 2) m
Base circle diameter
dg1 = z1 m cos α, dg2 = z2 m cos α
Circular pitch
p = π m
Normal pitch
t = π m cos α
Addendum
ha = m
Dedendum
hd = m + c = (1 + k) m
Whole depth
h = ha + hd = 2 m + c = (2 + k) m
Root diameter
df1 = (z1 – 2) m – 2 c = (z1 – 2 – 2k) m
df2 = (z2 – 2) m – 2 c = (z2 – 2 – 2k) m
Standard gears are widely used due to their simplicity, but for gears with a small number of teeth, there are inconveniences associated with strength, performance and tooth cutting. Generally, for pressure angle 14.5° full depth standard gears, number of teeth is above 26; for pressure angle 20° full depth standard gears, it is 14 or above; and for 20° stub tooth gears it is about 12 teeth or more. Below these numbers, it is safe to use profile shifted gears which will be discussed later.
Gear Module
Simply said, the gear terminology “module” refers to the gear tooth size. Larger tooth size enables more power to be transmitted, and it should of course be obvious that the mating gears must have the same module.
Supposing the case of spur gears, we would like to explain the technical aspects more precisely. As stated before, module is one type of units of tooth size measure, and multiplying a gear’s module by pi (π) yields the gear’s pitch. (Incidentally, in the context of screws and gears, the pitch is the distance between the consecutive threads and teeth.) Also, module is defined as the pitch diameter divided by the number of teeth, and the distance from the pitch circle to the outside circle (addendum) is equal to the module in spur gears.
In addition to module to indicate the tooth size, there are CP (circular pitch) used mainly to insure accurate positioning, DP (diametral pitch) in an inch based system, etc., but the most used unit internationally is module, and even in the US where DP is mostly used, the module based gears which are called metric gears are becoming more widespread. The units used in standardized gears are mostly module or DP, and at KHK they are also standardized but generally not too many standard CP gears are seen. Also, in module and CP units, as the values go up, the tooth size increases; but in DP unit, as the value increases, the tooth size become smaller.
In general gear technical papers and supplier provided gear related technical data, the explanations of these contents typically use spur gears as examples due to its simplicity and ease of understanding. We at KHK offer further explanation using spur gears on our website, and we urge you to refer to them.
Related links :
Gear Module – A detailed description of gear module
Metric Gears with pitch unit of module
Metric gears are used in many countries and areas where the main unit of measure is the metric system. The size of gear teeth is expressed by its pitch called module unit. On the other hand, countries such as US where the length unit is measured in inch, the unit of measure for gear pitch is usually in DP (diametral pitch).
The use of module as the gear pitch unit is specified by ISO. In many cases, module is the PCD (pitch circle diameter) divided by the number of teeth so that as the gear teeth get larger the module value goes up.
Besides module as the measure of tooth size, there are previously mentioned DP and CP (circular pitch). When CP is used and a pinion rotates once, the distance moved becomes an integer (mm) and is convenient in positioning in linear motion. Depending on the unit, whether module, DP or CP, a different hobbing tool is required.
The stock gears supplied by KHK are, with exception of CP gears, mostly module based metric gears and are widely used all over the world. On the other hand, KHK can also produce custom gears in module , CP and DP gears.
The following link will lead to the comparison of these pitches :
Table of Comparative Gear Pitch
Stainless Steel Spur Gears
For gear materials, SUS303 and SUS304 are most widely used. SUS304 is the most used stainless steel, but SUS303 has higher machinability.
Including SUS303 and SUS304, the following materials for stainless steel spur gears are generally used.
Austenite group stainless steel
SUS303, SUS304, SUS403
For heat treating, nitriding is used, but because the nitrided surface is shallow, tooth grinding is not doable.
Martensite group stainless steel
SUS420J, SUS440C, SUS403
For heat treating, tooth surface induction hardening is used, and tooth grinding is often performed following hardening.
Precipitation hardened group stainless steel
SUS630, SUS631
For heat treating, precipitation hardening is used, and tooth grinding is often used after hardening.
The anti-rust property is the characteristic of stainless steel spur gears. Therefore, their use is suitable where rust is unwanted such as in food, chemical and pharmaceutical machines.For example, if a pair of spur gears are made from a common plastic material, the gears tend to store heat, and the thermal expansion causes dimensional change resulting in reduction of backlash, so that if a plastic material is used for one side, it is recommended that a stainless is used for the other mating gear.
Types of KHK Stainless Steel Spur Gears :
SUS, SUSA, SUSL
Ground Spur Gears superior in quietness
Ground spur gears have gear surfaces ground using a gear grinder in order to achieve high precision and improve surface finish. Some of the reasons for using ground gears are improving noise and vibration, increasing torque transmission capacity, use in high rotational speed applications, etc.
In order to manufacture high strength and high precision gears, the deformation of tooth surface due to heat treating is normally corrected with teeth grinding operations. Also, to further improve precision of ground gears, in addition to teeth grinding, the gears are subjected prior to teeth grinding to grinding of the bore, outside diameter and flat surfaces. On the other hand, another way to improve the gear precision is to do the gear shaving operation prior to hear treating.
In general, since the use of ground gears are for the purpose of lowering noise and vibration, when mating a pair, if the one side is ground gear, a ground gear is used for the other side. But, when there is a difference in the number of teeth, in actual cases, the need for teeth grinding is determined case by case to optimize the balance between cost and strength, for example, by using ground gear only for the smaller gear.
Among the KHK brand of standard gears, there are many ground gears in various categories besides spur gears and we ask you to consider selecting them according to your application needs.
Types of KHK Ground Spur Gears :
MSGA, MSGB, SSGS, SSG
Spur gear catalog
Among the most frequently requested information for each gear in the spur gear catalogs are the following :
Dimension related data such as pitch diameter, outside diameter, face width, bore size, etc.
Tooth related information such as number of teeth and pitch (module)
Allowable torque from bending strength and surface durability
Precision class, also related to tooth grinding
Hardness related data with presence or absence of heat treatment and type
Photograph of the exterior view
Pricing
Gear’s Intended Use
First, one of the intended use of gears is to absorb the gap in torque or rotating speed between the prime movers such as motors and engines and the driven mechanisms.
When the prime mover has high rotation while the driven mechanism requires low rotation or when the prime mover generates torque not sufficient for the driven mechanism, the prime mover cannot be directly connected to the driven mechanism. In such cases, a speed reducer is utilized to absorb these gaps. Gears are often used in such speed reducers.
Also, for example, when two parallel rollers are used in rolling apparatus where the rollers turn synchronously, you can think of using servo motors on each roller, but if you can use spur gears, the two can be rotated with one motor using gears. (Because there is only one motor) the synchronization is naturally achieved. In other words, the use of spur gears here contributes to cost reduction while achieving change of rotational direction, synchronization of rollers and reduction in the number of motors.
Also, in mechanism design, gears can contribute to size reduction in the motor used in the application. When a certain speed and torque output is desired : (1) specify a motor that satisfies the conditions and use it directly
(2) we can think of using a motor with low torque and high speed than those of desired values but obtain the same output
by using gears. If the method in (2) can be used, (since generally, the low torque, high speed motor is compact), it is possible to downsize the motor in this application.
With screw gears and bevel gears, it is possible to produce complex power transmission such as changing the directions of the rotating shafts while reducing speed. For example, in a transport system, by using gears, it is possible to drive many shafts with one motor, allowing possible reduction in over-all cost by decreasing the number of motors.
There are many different intended use of gears beside the above examples, and in any event, needless to say, in transmission of power, it is important to select the best mechanical element suitable for each application.
Related links :
Gear Catalog - A detailed description of KHK’s gear catalog
Types of Gears - A detailed description of Types of Gears
Manufacturing of Spur Gears
The majority of metallic spur gears are made by cutting, but there are other methods such as rolling, casting and forging. On the other hand, for plastic spur gears, besides tooth cutting as with metal gears, injection molding may be used depending on the production quantity.
The production steps for metal spur gears, when high accuracy and hardness are not needed, usually consist of: cutting off round rod material, turning blanks on a lathe, tooth cutting using, for example, a hobbing machine, and deburring. In addition, set screw holes and keyways may be added as needed. Also, as far as the materials for spur gears are concerned, depending on the production lot, applications, etc. round rods and cast materials and others are used.
The tooth forming process can be roughly divided into two kinds as “gear form generating” and “gear form milling” methods. The gear form generating is a widely used method and its use allows mass production of high precision involute gears. In this method, by meshing the cutting tool with the gear blank, the tooth form equal to the pitch of the cutter is generated. As for kinds of cutting tools, they can be classified as method using a hob (cylindrical twisted tooth rotating cutter), method using rack shaped cutter and method using pinion shaped cutter. On the other hand, the gear form milling method consists of using a cutter shaped as the gear groove form attached to a milling machine and cutting one groove of the gear at a time.
After gear cutting, tooth surface grinding and shaving processes are sometimes used to increase accuracy and strength.
Gear grinding is used to improve the accuracy and surface roughness of gears. It indicates the process of grinding the gear tooth surface, after tooth cutting and heat treating, with a polishing stone which is rotating at high speed. It is performed on a special tooth grinding machine. The production process steps for ground gears can be listed, for one example, as cutting off material, blank making using lathe, tooth cutting, deburring, heat treating, side surface grinding, bore grinding, and tooth grinding. On the other hand, shaving means a process used in high rate production of a large quantity of the same gears such as in the automotive industry where it is used after gear cutting to further improve gear accuracy. However, the shaving cutter used in the process lacks versatility, requiring different ones in many cases for each different gear. Ordinarily after shaving, gears are heat treated and finished with honing."
Spur gears are the most easily visualized common gears that transmit motion between two parallel shafts. Because of their shape, they are classified as a type of cylindrical gears. Since the tooth surfaces of the gears are parallel to the axes of the mounted shafts, there is no thrust force generated in the axial direction. Also, because of the ease of production, these gears can be made to a high degree of precision. On the other hand, spur gears have a disadvantage in that they easily make noise. Generally speaking, when two spur gears are in mesh, the gear with more teeth is called the “gear” and the one with the smaller number of teeth is called the “pinion”.
The unit to indicate the sizes of spur gears is commonly stated, as specified by ISO, to be “module”. In recent years, it is usual to set the pressure angle to 20 degrees. In commercial machinery, it is most common to use a portion of an involute curve as the tooth profile.
Even though not limited to spur gears, profile shifted gears are used when it is necessary to adjust the center distance slightly or to strengthen the gear teeth. They are produced by adjusting the distance between the gear cutting tool called the hobbing tool and the gear in the production stage. When the shift is positive, the bending strength of the gear increases, while a negative shift slightly reduces the center distance. The backlash is the play between the teeth when two gears are meshed and is needed for the smooth rotation of gears. When the backlash is too large, it leads to increased vibration and noise while the backlash that is too small leads to tooth failure due to the lack of lubrication.
All KHK spur gears have an involute tooth shape. In other words, they are involute gears using part of the involute curve as their tooth forms. Looking generally, the involute shape is the most wide-spread gear tooth form due to, among other reasons, the ability to absorb small center distance errors, easily made production tools simplify manufacturing, thick roots of the teeth make it strong, etc. Tooth shape is often described as a specification in drawing of a spur gear as indicated by the height of teeth. In addition to standard full depth teeth, extended addendum and stub tooth profiles exist.
