| Q & A concerning Gears
Q & A concerning Gears
¦ Questions on the basics of Gears
1) What is "Module"?
2) What is "Reference diameter"?
3) What is "Pressure angle"?
4) What is the difference between "single thread" and "double threads" in worms?
5) How do you distinguish the right-hand teeth (R) from left-hand teeth (L)?
6) What is the difference between "Module (M)" and "Circular pitch (CP)"?
7) What is "Backlash"?
8) What are "Bending strength" and "Surface durability" respetively?
9) When selecting gears based on use, which do we hve to consider mainly, the bending strength or the surface durability?
10) Direction of spiral or hand of helix, and direction of thrust.
11) What is "Center distance"?
12) When designing the casing of gearboxes, for instance, how do you decide the center distance of the spur gears taking the tolerance into consideration?
13) What is "Shaft angle"?
14) What is "Locating distance"?
15) In the dimensional tolerance of the locating distance how well do you decide the exact dimension?
16) How much can you raise the strength by quenching?
17) How do you calculate the reference diameter of DP (diametral pitch) system spur gears? (Example: DP8-15z)
18) What is "Crowning"?
19) What does "Gear tooth modifications" mean exactly?
20) What is "Semitopping"?
¦ Questions concerning KHK stock gears
1) What is the difference between MSGA ground spur gear and MSGB ground spur gear?
2) Which type of gears are suitable for high speed rotation?
3) Where can you purchase KHK stock gears? (Where can you get quotations from?)
4) Are tooth areas of SS(A) and SR(F) hardened by means of quenching?
5) When a SSA2.5-40 makes a half turn, what will be the distance of travel, in millimeters, of the mating rack?
6) Does KHK carry SS2-20 spur gears without bore?
7) Is it possible to use SSG ground spur gears in the atmosphere where a temperature can rise to 300 °C ?
8) What is the difference between transverse module KHG helical gears and normal module SH helical gears?
9) Why has MC901 (PS, PM, PB, etc.) changed in its color?
10) Is there any decrease in the strength of MC901 due to change of the coloring agent?
11) Is there any rules as to the direction of rotation for a pair of spiral miter gears, SMS3-25R/L?
12) Are there any means to adjust backlash to a minimum?
13) Is it possible to use duplex worm gear pair at zero backlash?
14) Which type of gears are quiet?
15) Which type of gears are resistant to rusting?
16) How much can you raise the strength by quenching?
¦Q & A on the basics of Gear
1) What is "Module"?
- Module is the unit of size that indicates how big or small a gear is.
The module is the length of pitch diameter per tooth.
To represent the tooth size CP and DP are also used.
CP stands for circular pitch, and DP stands for diametral pitch.)
*Reference* "Pitch" is the distance between corresponing points on adjacent teeth.
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2) What is "Reference diameter"?
- The reference diameter is the diameter of the pitch circle of a gear. It is also known as "Pitch diameter." The two essential factors in determining the size of a gear are the number of teeth and the module. Then the reference diameter is:
Number of teeth x Transverse module
JIS gear terms for "reference diameter" before the amendment was "pitch diameter."
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3) What is "Pressure angle"?
- Pressure angle is the angle that the line of force makes with a line at right angles to the center line of two gears at the pitch points. Pressure angle is also referred to as "tooth shape."
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4) What is the difference between "single thread" and "double threads" in worms?
- The thread in this instance is a continuous helical rib on a worm. The single thread means that a worm has one thread. The double threads means that a worm has two threads.The thread is the equivalent of the gear tooth.
The larger the number of thread, the wider the lead angle.
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5) How can you distinguish right-hand gear from left-hand gear?
- When the gear is placed on a flat surface, the teeth of a left-hand gear lean to the left and the teeth of a right-hand gear lean to the right.
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6) What is the difference between "Module (M)" and "Circular pitch (CP)"?
- Circular pitch is the pitch or distance, measuring along reference surface, from any point on a gear tooth to the corresponding point on the next tooth in millimeters.
The number of CP devided by pi ( p) equals the number of module -
CP and M, the unit of which is also in millimeters, the system of unit used is also the metric system, are related by the expression:
M = CP/p
Both module and circular pitch describe the size of a gear.
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7) What is "Backlash"?
- Backlash is the clearance space between the teeth of the mating gears.
It is necessary for smooth operation of the gears.
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8) What is the difference between "Bending strength" and "Surface durability"?
- Bending strength is the strength of the gear to carry the dynamic loads sufficiently without breaking or shearing. Surface durability is the surface-endurance properties of the gear to carry the dynamic loads without excessive wear.
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9) When selecting gears based on use, which do we have to consider mainly, the bending strength or the surface durability?
- In general, both the bending strength and the surface durability are to be considered.
How-ever, there are some cases where the bending strength is more important.
Such cases are:
a) the frequency of gear use is low,
b) gears are operated by hand without the help of electricity,
c) gears to mesh at a low speed. In any case, the decision has to be made by the design engineer.
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10) Direction of spiral (or hand of helix), and direction of thrust.
- The meshed gears develop, under operating conditions, thrust force. The only exception is spur gears, where the teeth are straight and parallel to shaft axis.
For relations between the direction of spiral and the direction of thrust on each type of gear, please refer to KHK3009 catalog:
* For helical gears and helical racks* Click here
* For spiral miter gears* Click here
* For spiral bevel gears* Click here
In the case of straight bevel gears, the thrust is always backward toward the hub.
* For screw gears* Click here
* For worm gear pair* Click here
As to the magnitude of the gear forces* Click here
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11) What is "Center distance"?
- The center distance is the distance between the center of the shaft of one gear to the center of the shaft of the other gear.
The backlash is inseparably related to the center distance;
A positive permissible deviation of the center distance means a larger backlash, and a negative permissible deviation of the center distance means a smaller backlash.
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12) When designing the casing of gearboxes, for instance, how do you decide the center distance of the spur gears taking the tolerance into consideration?
- Generally speaking, the center distance is set up to the pertinent specified values, or purposely set up to a plus-tolerance or minus- tolerance depending on the case.
Please refer to JGMA standards 1101-01 (2000), which specifies the tolerance of the center distance for spur gears.
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13) What is "Shaft angle"?
- Intersecting axes gears (bevel gears), and nonparallel & nonintersecting axes gears (screw gears and worm gear pair) connect from one shaft to another. The angle of these two shafts is called the shaft angle. The angle is normally 90° . Setting up the shaft angle accurately as per specifications is very important as it affects the tooth contact and the backlash.
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14) What is "Locating distance"?
- The locating distance is the linear dimension from the axial locating surface of a bevel gear to the intersection point of its axis with the axis of its mate.
The locating distance is the very im-portant dimension that influence the tooth contact and the backlash.
* Tip: The locating distance is also called as "mounting distance."
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15) In the dimensional tolerance of the locating distance how well do you decide the exact dimension?
- It will be the best way to aim at near-zero for the proper backlash and tooth contact.
It is recommended that you aim at the specified dimensional standard (zero) up to JS7 ~ JS9.
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16) What is "Zerol Bevel Gear"?
- Zerol bevel gear is a special case of spiral bevel gear.
It is a spiral bevel with zero degree of spiral angle tooth advance.
It has the characteristics of both the straight and spiral bevel gears.
Advantages:
- The forces acting upon the tooth are the same as for a straight bevel gear.
- Has higher strength, and is quieter, compared with a straight bevel gears.
- The teeth are grindable, which means it is possible to produce high accuracy products.
* Tip: The spiral angle of spiral bevel gear is 35° generally.
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17) How do you calculate the reference diameter of DP (diametral pitch) system spur gears? -------- Example: DP8-15z
- Convert the diametral pitch into the module;
Module = 25.4/DP (diametral pitch) -------- 25.4/8 = 3.175 mm
Then calculate the reference diameter;
da = 3.175 x 15 = 47.625 mm
*Reference* 1 inch = 25.4 mm
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18) What is "Crowning"?
- Crowning is the removal of a slight amount of tooth from the center on out to reach edge, making the tooth surface slightly convex.
This method allows the gear to maintain contact in the central region of the tooth and permits avoidance of edge contact.
The larger the amount of crowning, the less the amount of tooth surface contact.
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19) What does "Gear tooth modifications" mean?
- It means the intentional deviations from the proper tooth profile to avoid excessive tooth load deflection interference by chamfering the tooth surface. It is similar to crowning except that it is a simple process and only an approximation to crowning.
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20) What is "Semitopping"?
- Semitopping is the chamfering of the tooth's top corner, which is accomplished simultane-ously with tooth generation.
The effects:
- The semitopped gear has no burr on the tips of gear teeth.
- It helps to prevent surface damage on gears that tends to done during handling and transpor-tation.
*Note* Here the tooth's top corner and the tips of gear teeth are used in the same meaning.
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Q & A concerning KHK stock gears
1) What is the difference between MSGA ground spur gear and MSGB ground spur gear?
- The entire specifications are the same except for the size of bore.
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2) Which type of gears are suitable for high speed rotation?
- The gears that are strong enough to withstand high speed rotation are suitable. The quiet gears are also suitable. That is to say,
- High-precision ground tooth gears
Ground tooth gears are better in their accuracy than cut or hobbed gears, and therefore, they are stronger and quieter.
KHK carries stock ground tooth gears for the following types of gears: spur gear, rack, helical gear, helical rack, spiral miter gear, spiral bevel gear, worm gear pair.
- Gears with spiral teeth
Gears with spiral teeth have better contact ratio than straight teeth gears, and therefore, they are stronger and quieter.
It is recommended that you choose helical gears in preference to spur gears, and spiral bevel gears to stright bevel gears.
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3) Where can you purchase KHK stock gears? (Where can you get quotations from?)
- You can purchase them from KHK representative in your country. As to the whereabouts of KHK overseas representatives -------- *Click here
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4) Are tooth areas of SS(A) and SR(F) hardened by means of quenching?
- No. These products are not heat treated.
Among spur gears SSG ground spur gears are the ones tooth areas are hardened.
MSGA(B) ground spur gears are the ones whole part of gears are hardened by means of carburizing/quenching.
Among racks SRGF ground racks are the ones tooth areas are hardened.
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5) When a SSA2.5-40 makes a half turn, what will be the distance of travel in millimeters of the mating rack?
- The movement of the rack in a half turn means a half of 40 teeth or 20 teeth (20 pitches) move forward. Thus, the distance the rack moves in millimeter is:
1 pitch x No. of teeth = (pi×2.5)×20=157.08mm
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6) Does KHK carry SS2-20 spur gears without bore?
- No. There is no SS2-20 without bore in the line of KHK stock gears.
However, it might be possible to produce the SS2-20 without bore as special made-to-order gear.
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7) Is it possible to use SSG ground spur gears in the atmosphere where a temperature can rise to 300 °C?
- No, it's not possible. SSG ground spur gears are made out of S45C and their tooth area is high-frequency Induction Hardened.
Because of this, If SSG ground spur gears are exposed to the temperature of 300 °C, their hardness will drop and their stength will be lost. If the temper-ature is below 150 °C, there will be no such ill effect.
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8) What is the difference between transverse module KHG helical gears and normal module SH helical gears?
- The difference is in the reference section of these gears. And their advantages/disadvantages are:
Transverse module KHG
Advantage ...As the values of reference diameter and center distance of a KHG correspond to that of a spur gear, replacement of a helical gear from a spur gear can be made readily without changing a gearbox.
Disadvantage from the viewpoint of manufacture···The special hob or cutter is necessary.
Normal module SH
Advantage from the viewpoint of manufacture···The hobs or cutters for a spur gear can be used for a SH helical gear. No special hob or cutter is necessary, which results in reasonable costs.
Disadvantage···It will be necessary to make a new gearbox as the center distances of a SH and a spur gear are different.
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9) Why has MC901 (PS, PM, PB, etc.) changed in its color?
- The former MC901 used a coloring agent that contained a small amount of heavy metals (cobalt and chrome).
The coloring agent now used in the new MC901 does not contain these heavy metals.
With this change of the coloring agent, the hue of MC901 has been changed.
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10) Is there any decrease in the strength of MC901 due to change of the coloring agent?
- There is no decrease in the strength.
KHK has the written statement from Nippon Polypenco Limited to the effect that there is no change in the mechanical property and machinability between the new and previous products.
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11) Is there any rules as to the direction of rotation for a pair of spiral miter gears, SMS3-25R/L?
- They are usable for both clockwise and counterclockwise directions.
However, care must be taken of the axial thrust forces.
With spiral miter gears there is a backward thrust on one gear and a forward thrust on the mating gear, depending upon the direction of rotation and gear hand. It is necessary, therefore, that the thrust bearings must be selected properly to sustain these axial loads and prevent axial movement of the loaded shaft.
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12) Are there any means to adjust backlash to a minimum?
- KHK does not have any stock gears that can adjust backlash at zero.
However, KHK carry items that allow you to adjust the amount of baklash to a minimum. These are "Tapered Racks and Pinions" and "Duplex Worms and Worm Wheels". In any case, it is not recom-mended that you adjust the backlash at zero.
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13) Is it possible to use duplex worm gear pair at zero backlash?
- Basically, it is not possible.
There are possibilities that the gear pair does not turn well. It is possible that backlash change at certain point during a turn due to runout error, which causes a jam.
Backlash is necessary for smooth operation of the gears.
Too small amount of backlash will create the problems:
·Abnormal wear of the gear due to a lack of space for lubrication.
·Deformation of the gear due to excessive heat and a consequent jam of the gear due to zero-backlash condition.
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14) Which type of gears are quiet?
- The high precision gears with ground finish are quiet.
The use of helical gears in preference to spur gears is more effective in reducing noise level since the teeth of helical gears are cut at an angle to the hole (axis), which increases the contact ratio.
Plastic gears are also quiet. (However, they are less strong.)
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15) Which type of gears are resistant to rusting?
- Stainless steel and plastic gears are good against rusting.
Plastic gears are able to operate with minimum or no lubrication, due to inherent lubricity.
However, using plastic against plastic is not desirable since they tend to heat up and expand resulting in decrease of backlash. It is desirable that you mate a plastic gear with a metal gear.
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16) How much can you raise the strength by quenching?
- For gears made out of S45C such as SS spur gears, if they are high frequency induction heat treated, the tooth surface strength increases about four-fold.
On the other hand, the precision grade such as pitch error will drop one grade.
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