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Elements of Metric Gear Technology - Page 5
Elements of Metric Gear Technology
9.1 Worm Mesh Geometry
Although the worm tooth form can be of a variety, the most popular is equivalent to a V-type screw thread, as in
. The mating worm gear teeth have a helical lead. (Note: The name "worm wheel" is often used interchangeably with "worm gear".) A central section of the mesh, taken through the worm's axis and perpendicular to the worm gear's axis, as shown in
, reveals a rack-type tooth of the worm, and a curved involute tooth form for the worm gear. However, the involute features are only true for the central section. Sections on either side of the worm axis reveal nonsymmetric and noninvolute tooth profiles. Thus, a worm gear mesh is not a true involute mesh. Also, for conjugate action, the center distance of the mesh must be an exact duplicate of that used in generating the worm gear.
To increase the length-of-action, the worm gear is made of a throated shape to wrap around the worm.
9.1.1 Worm Tooth Proportions
Worm tooth dimensions, such as addendum, dedendum, pressure angle, etc., follow the same standards as those for spur and helical gears. The standard values apply to the central section of the mesh. See
. A high pressure angle is favored and in some applications values as high as 25° and 30° are used.
9.1.2 Number Of Threads
The worm can be considered resembling a helical gear with a high helix angle.
For extremely high helix angles, there is one continuous tooth or thread. For slightly smaller angles, there can be two, three or even more threads. Thus, a worm is characterized by the number of threads,
9.1.3 Pitch Diameters, Lead and Lead Angle
9.1.4 Center Distance
9.2 Cylindrical Worm Gear Calculations
Cylindrical worms may be considered cylindrical type gears with screw threads. Generally, the mesh has a 90° shaft angle. The number of threads in the worm is equivalent to the number of teeth in a gear of a screw type gear mesh.
Thus, a one-thread worm is equivalent to a one-tooth gear; and two-threads equivalent to two-teeth, etc. Referring to
, for a lead angle ɣ, measured on the pitch cylinder, each rotation of the worm makes the thread advance one lead.
There are four worm tooth profiles in JIS B 1723, as defined below.
Type I Worm:
This worm tooth profile is trapezoid in the radial or axial plane.
Type II Worm:
This tooth profile is trapezoid viewed in the normal surface.
Type III Worm:
This worm is formed by a cutter in which the tooth profile is trapezoid form viewed from the radial surface or axial plane set at the lead angle. Examples are milling and grinding profile cutters.
Type IV Worm:
This tooth profile is involute as viewed from the radial surface or at the lead angle. It is an involute helicoid, and is known by that name.
Type III worm is the most popular. In this type, the normal pressure angle α
has the tendency to become smaller than that of the cutter, α
Per JIS, Type III worm uses a radial module
and cutter pressure angle α
= 20° as the module and pressure angle. A special worm hob is required to cut a Type III worm gear. Standard values of radial module,
, are presented in
Because the worm mesh couples nonparallel and nonintersecting axes, the radial surface of the worm, or radial cross section, is the same as the normal surface of the worm gear.
Similarly, the normal surface of the worm is the radial surface of the worm gear. The common surface of the worm and worm gear is the normal surface. Using the normal module,
, is most popular. Then, an ordinary hob can be used to cut the worm gear.
presents the relationships among worm and worm gear radial surfaces, normal surfaces, axial surfaces, module, pressure angle, pitch and lead.
can help the understanding of the relationships in
. They are similar to the relations in
that the helix angle β be substituted by (90° – ɣ). We can consider that a worm with lead angle ɣ is almost the same as a screw gear with helix angle (90° – ɣ).
9.2.1 Axial Module Worm Gears
presents the equations, for dimensions shown in
, for worm gears with axial module, m
, and normal pressure angle α
9.2.2 Normal Module System Worm Gears
The equations for normal module system worm gears are based on a normal module,
, and normal pressure angle, α
= 20°. See
9.3 Crowning Of The Worm Gear Tooth
Crowning is critically important to worm gears (worm wheels). Not only can it eliminate abnormal tooth contact due to incorrect assembly, but it also provides for the forming of an oil film, which enhances the lubrication effect of the mesh. This can favorably impact endurance and transmission efficiency of the worm mesh. There are four methods of crowning worm gears:
1. Cut Worm Gear With A Hob Cutter Of Greater Pitch Diameter Than The Worm.
A crownless worm gear results when it is made by using a hob that has an identical pitch diameter as that of the worm. This crownless worm gear is very difficult to assemble correctly. Proper tooth contact and a complete oil film are usually not possible.
However, it is relatively easy to obtain a crowned worm gear by cutting it with a hob whose pitch diameter is slightly larger than that of the worm. This is shown in
. This creates teeth contact in the center region with space for oil film formation.
2. Recut With Hob Center Distance Adjustment.
The first step is to cut the worm gear at standard center distance. This results in no crowning. Then the worm gear is finished with the same hob by recutting with the hob axis shifted parallel to the worm gear axis by ±Δ
. This results in a crowning effect, shown in
3. Hob Axis Inclining
From Standard Position.
In standard cutting, the hob axis is oriented at the proper angle to the worm gear axis. After that, the hob axis is shifted slightly left and then right, Δθ, in a plane parallel to the worm gear axis, to cut a crown effect on the worm gear tooth. This is shown in
. Only method 1 is popular. Methods 2 and 3 are seldom used.
4. Use A Worm With A Larger Pressure Angle Than The Worm Gear.
This is a very complex method, both theoretically and practically. Usually, the crowning is done to the worm gear, but in this method the modification is on the worm. That is, to change the pressure angle and pitch of the worm without changing the pitch line parallel to the axis, in accordance with the relationships shown in
In order to raise the pressure angle from before change, α
', to after change, α
, it is necessary to increase the axial pitch,
', to a new value,
. The amount of crowning is represented as the space between the worm and worm gear at the meshing point A in
This amount may be approximated by the following equation:
= Pitch diameter of worm
= Factor from
= Axial pitch after change
' = Axial pitch before change
An example of calculating worm crowning is shown in
Because the theory and equations of these methods are so complicated, they are beyond the scope of this treatment. Usually, all stock worm gears are produced with crowning.
9.4 Self-Locking Of Worm Mesh
Self-locking is a unique characteristic of worm meshes that can be put to advantage. It is the feature that a worm cannot be driven by the worm gear. It is very useful in the design of some equipment, such as lifting, in that the drive can stop at any position without concern that it can slip in reverse. However, in some situations it can be detrimental if the system requires reverse sensitivity, such as a servomechanism.
Self-locking does not occur in all worm meshes, since it requires special conditions as outlined here. In this analysis, only the driving force acting upon the tooth surfaces is considered without any regard to losses due to bearing friction, lubricant agitation, etc. The governing conditions are as follows:
= tangential driving force of worm
= normal pressure angle
ɣ = lead angle of worm
µ = coefficient of friction
= normal driving force of worm
> 0 then there is no self-locking effect at all. Therefore,
≤ 0 is the critical limit of self-locking.
in Equation (9-6) be 20°, then the condition:
≤ 0 will become:
(cos 20° sin ɣ – µ cos ɣ) ≤ 0
shows the critical limit of self-locking for lead angle ɣ and coefficient of friction µ. Practically, it is very hard to assess the exact value of coefficient of
friction µ. Further, the bearing loss, lubricant agitation loss, etc. can add many side effects. Therefore, it is not easy to establish precise self-locking conditions. However, it is true that the smaller the lead angle ɣ, the more likely the self-locking condition will occur.
SECTION 10: TOOTH THICKNESS
There are direct and indirect methods for measuring tooth thickness. In general, there are three methods:
Chordal Thickness Measurement
Over Pin or Ball Measurement
10.1 Chordal Thickness Measurement
This method employs a tooth caliper that is referenced from the gear's outside diameter. Thickness is measured at the pitch circle. See
10.1.1 Spur Gears
presents equations for each chordal thickness measurement.
10.1.2 Spur Racks And Helical Racks
The governing equations become simple since the rack tooth profile is trapezoid, as shown in
10.1.3 Helical Gears
The chordal thickness of helical gears should be measured on the normal surface basis as shown in
presents the equations for chordal thickness of helical gears in the radial system. See
for Gleason straight bevel gears.
presents equations for chordal thickness of a standard straight bevel gear.
If a standard straight bevel gear is cut by a Gleason straight bevel cutter, the tooth angle should be adjusted according to:
This angle is used as a reference in determining the circular tooth thickness,
, in setting up the gear cutting machine.
presents equations for chordal thickness of a Gleason spiral bevel gear.
shows how to determine the circular tooth thickness factor
for Gleason spiral bevel gears.
The calculations of circular thickness of a Gleason spiral bevel gear are so complicated that we do not intend to go further in this presentation.
10.1.5 Worms And Worm Gears
presents equations for chordal thickness of axial module worms and worm gears.
contains the equations for chordal thickness of normal module worms and worm gears.
10.2 Span Measurement Of Teeth
Span measurement of teeth,
, is a measure over a number of teeth,
, made by means of a special tooth thickness micrometer. The value measured is the sum of normal circular tooth thickness on the base circle,
, and normal pitch,
10.2.1 Spur And Internal Gears
The applicable equations are presented in
shows the span measurement of a spur gear. This measurement is on the outside of the teeth. For internal gears the tooth profile is opposite to that of the external spur gear. Therefore, the measurement is between the inside of the tooth profiles.
10.2.2 Helical Gears
present equations for span measurement of the normal and the radial systems, respectively, of helical gears.
There is a requirement of a minimum blank width to make a helical gear span measurement. Let
be the minimum value for blank width. Then
is the helix angle at the base cylinder,
From the above, we can determine that at least 3mm of Δ
is required to make a stable measurement of
10.3 Over Pin (Ball) Measurement
As shown in
, measurement is made over the outside of two pins that are inserted in diametrically opposite tooth spaces, for even tooth number gears; and as close as possible for odd tooth number gears.
The procedure for measuring a rack with a pin or a ball is as shown in
by putting pin or ball in the tooth space and using a micrometer between it and a reference surface. Internal gears are similarly measured, except that the measurement is between the pins. See
. Helical gears can only be measured with balls. In the case of a worm, three pins are used, as shown in
. This is similar to the procedure of measuring a screw thread. All these cases are discussed in detail in the following sections.
Note that gear literature uses "over pins" and "over wires" terminology interchangeably. The "over wires" term is often associated with very fine pitch gears because the diameters are accordingly small.
10.3.1 Spur Gears
In measuring a gear, the size of the pin must be such that the over pins measurement is larger than the gear's outside diameter. An ideal value is one that would place the point of contact (tangent point) of pin and tooth profile at the pitch radius. However, this is not a necessary requirement. Referring to
, following are the equations for calculating the over pins measurement for a specific tooth thickness,
, regardless of where the pin contacts the tooth profile:
For even number of teeth:
For odd number of teeth:
where the value of φ
is obtained from:
When tooth thickness,
, is to be calculated from a known over pins measurement,
, the above equations can be manipulated to yield:
In measuring a standard gear, the size of the pin must meet the condition that its surface should have the tangent point at the standard pitch circle. While, in measuring a shifted gear, the surface of the pin should have the tangent point at the
The ideal diameters of pins when calculated from the equations of
may not be practical. So, in practice, we select a standard pin diameter close to the ideal value. After the actual diameter of pin
is determined, the over pin measurement
can be calculated from
is a dimensional table under the condition of module
= 1 and pressure angle α = 20° with which the pin has the tangent point at
10.3.2 Spur Racks And Helical Racks
In measuring a rack, the pin is ideally tangent with the tooth flank at the pitch line. The equations in
can, thus, be derived. In the case of a helical rack, module
, and pressure angle α, in
, can be substituted by normal module,
, and normal pressure angle, α
, resulting in
» Continued on page 6
Section 1: Introduction to Metric Gears
Section 2: Introduction to Gear Technology
Section 3: Details of Involute Gearing
Section 4: Spur Gear Calculations
Section 5: Internal Gears
Section 6: Helical Gears
Screw Gear or Crossed Helical Gear Meshes
Section 8: Bevel Gearing
Section 9: Worm Mesh
Section 10: Tooth Thickness
Section 11: Contact Radio
Section 12: Gear Tooth Modications
Section 13: Gear Trains
Section 14: Backlash
Section 15: Gear Accuracy
Section 16: Gear Forces
Section 17: Strength and Durability of Gears
Section 18: Design of Plastic Gears
Section 19: Features of Tooth Surface Contact
Section 20: Lubrication of Gears
Section 21: Gear Noise
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