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Molded vs. Cut Plastic Gears, Elements of Metric Gear Technology, Page 11
Molded vs. Cut Plastic Gears
, Elements of Metric Gear Technology (Cont.)
Inserts are usually produced by screw machines and made of aluminum or brass. It is advantageous to attempt to match the coefficient of thermal expansion of the plastic to the materials used for inserts. This will reduce the residual stresses in the plastic part of the gear during contraction while cooling after molding.
When metal inserts are used, generous radii and fillets in the plastic gear are recommended to avoid stress concentration. It is also possible to use other types of metal inserts, such as self-threading, self-tapping screws, press fits and knurled inserts. One advantage of the first two of these is that they permit repeated assembly and disassembly without part failure or fatigue.
18.6.5 Attachment Of Plastic Gears to Shafts
Several methods of attaching gears to shafts are in common use. These include splines, keys, integral shafts, set screws, and plain and knurled press fits.
lists some of the basic characteristics of each of these fastening methods.
Depending on the application, plastic gears can operate with continuous lubrication, initial lubrication, or no lubrication. According to L.D. Martin ("Injection Molded Plastic Gears", Plastic Design and Processing, 1968; Part 1, August, pp 38-45; Part 2, September, pp. 33-35):
All gears function more effectively with lubrication and will have a longer service life.
A light spindle oil (SAE 10) is generally recommended as are the usual lubricants; these include silicone and hydrocarbon oils, and in some cases cold water is acceptable as well.
Under certain conditions, dry lubricants such as molybdenum disulfide, can be used to reduce tooth friction.
Ample experience and evidence exist substantiating that plastic gears can operate with a metal mate without the need of a lubricant, as long as the stress levels are not exceeded. It is also true that in the case of a moderate stress level, relative to the materials rating, plastic gears can be meshed together without a lubricant. However, as the stress level is increased, there is a tendency for a localized plastic-to-plastic welding to occur, which increases friction and wear. The level of this problem varies with the particular type of plastic.
A key advantage of plastic gearing is that, for many applications, running dry is adequate. When a situation of stress and shock level is uncertain, using the proper lubricant will provide a safety margin and certainly will cause no harm. The chief consideration should be in choosing a lubricant's chemical compatibility with the particular plastic. Least likely to encounter problems with typical gear oils and greases are: nylons, Delrins (acetals), phenolics, polyethylene and polypropylene. Materials requiring caution are: polystyrene, polycarbonates, polyvinyl chloride and ABS resins. An alternate to external lubrication is to use plastics fortified with a solid state lubricant. Molybdenum disulfide in nylon and acetal are commonly used. Also, graphite, colloidal carbon and silicone are used as fillers.
In no event should there be need of an elaborate sophisticated lubrication system such as for metal gearing. If such a system is contemplated, then the choice of plastic gearing is in question. Simplicity is the plastic gear's inherent feature.
18.6.7 Molded vs. Cut Plastic Gears
Although not nearly as common as the injection molding process, both thermosetting and thermoplastic plastic gears can be readily machined. The machining of plastic gears can be considered for high precision parts with close tolerances and for the development of prototypes for which the investment in a mold may not be justified. Standard stock gears of reasonable precision are produced by using blanks molded with brass inserts, which are subsequently hobbed to close tolerances.
When to use molded gears vs. cut plastic gears is usually determined on the basis of production quantity, body features that may favor molding, quality level and unit cost. Often, the initial prototype quantity will be machine cut, and investment in molding tools is deferred until the product and market is assured. However, with some plastics this approach can encounter problems.
The performance of molded vs. cut plastic gears is not always identical. Differences occur due to subtle causes. Bar stock and molding stock may not be precisely the same. Molding temperature can have an effect. Also, surface finishes will be different for cut vs. molded gears. And finally, there is the impact of shrinkage with molding which may not have been adequately compensated.
18.6.8 Elimination of Gear Noise
Incomplete conjugate action and/or excessive backlash are usually the source of noise. Plastic molded gears are generally less accurate than their metal counterparts. Furthermore, due to the presence of a larger Total Composite Error, there is more backlash built into the gear train.
To avoid noise, more resilient material, such as urethane, can be used.
shows several gears made of urethane which, in mesh with Delrin gears, produce a practically noiseless gear train. The face width of the urethane gears must be increased correspondingly to compensate for lower load carrying ability of this material.
18.7 Mold Construction
Depending on the quantity of gears to be produced, a decision has to be made to make one single cavity or a multiplicity of identical cavities. If more than one cavity is involved, these are used as "family molds" inserted in mold bases which can accommodate a number of cavities for identical or different parts. Since special terminology will be used, we shall first describe the elements shown in
is the element which assures the proper location of the mold on the platen with respect to the nozzle which injects the molten plastic.
is the element which mates with the nozzle. It has a spherical or flat receptacle which accurately mates with the surface of the nozzle.
is the channel in the sprue bushing through which the molten plastic is injected.
is the channel which distributes material to different cavities within the same mold base.
is the element which, by its presence, restricts the flow of plastic; hence, a hole or void will be created in the molded part.
are operated by the molding machine. These have a relative motion with respect to the cavity in the direction which will cause ejection of the part from the mold.
is considered the side on which the sprue bushing and the nozzle are located.
is the orifice through which the molten plastic enters the cavity.
(not visible due to its small size) is a minuscule opening through which the air can be evacuated from the cavity as the molten plastic fills it. The vent is configured to let air escape, but does not fill up with plastic.
The location of the gate on the gear is extremely important. If a side gate is used, as shown in
, the material is injected in one spot and from there it flows to fill out the cavity. This creates a weld line opposite to the gate. Since the plastic material is less fluid at that point in time, it will be of limited strength where the weld is located. Furthermore, the shrinkage of the material in the direction of the flow will be different from that perpendicular to the flow. As a result, a side-gated gear or rotating part will be somewhat elliptical rather than round.
In order to eliminate this problem, "diaphragm gating" can be used, which will cause the injection of material in all directions at the same time
. The disadvantage of this method is the presence of a burr at the hub and no means of support of the core pin because of the presence of the sprue.
Figure 18-14 cont.
Figure 18-14 cont.
The best, but most elaborate, way is "multiple pin gating"
. In this case, the plastic is injected at several places symmetrically located. This will assure reasonable viscosity of plastic when the material welds, as well as create uniform shrinkage in all directions. The problem is the elaborate nature of the mold arrangement – so called 3-plate molds, in
18-14 part 2
18-14 part 3
– accompanied by high costs. If precision is a requirement, this way of molding is a must, particularly if the gears are of a larger diameter.
To compare the complexity of a 3-plate mold with a 2-plate mold, which is used for edge gating,
can serve as an illustration.
SECTION 19: FEATURES OF TOOTH SURFACE CONTACT
Tooth surface contact is critical to noise, vibration, efficiency, strength, wear and life. To obtain good contact, the designer must give proper consideration to the following features:
Modifying the Tooth Shape
Improve tooth contact by crowning or relieving.
Using Higher Precision Gear
Specify higher accuracy by design. Also, specify that the manufacturing process is to include grinding or lapping.
Controlling the Accuracy of the Gear Assembly
Specify adequate shaft parallelism and perpendicularity of the gear housing (box or structure).
Surface contact quality of spur and helical gears can be reasonably controlled and verified through piece part inspection. However, for the most part, bevel and worm gears cannot be equally well inspected. Consequently, final inspection of bevel and worm mesh tooth contact in assembly provides a quality criterion for control. Then, as required, gears can be axially adjusted to achieve desired contact.
JIS B 1741 classifies surface contact into three levels, as presented in
. The percentage in
considers only the effective width and height of teeth.
19.1 Surface Contact Of Spur And Helical Meshes
A check of contact is, typically, only done to verify the accuracy of the installation, rather than the individual gears. The usual method is to blue dye the gear teeth and operate for a short time. This reveals the contact area for inspection and evaluation.
19.2 Surface Contact Of A Bevel Gear
It is important to check the surface contact of a bevel gear both during manufacturing and again in final assembly. The method is to apply a colored dye and observe the contact area after running. Usually some load is applied, either the actual or applied braking, to realize a realistic contact condition. Ideal contact favors the toe end under no or light load, as shown in
; and, as load is increased to full load, contact shifts to the central part of the tooth width.
Even when a gear is ideally manufactured, it may reveal poor surface contact due to lack of precision in housing or improper mounting position, or both. Usual major faults are:
Shafts are not intersecting, but are skew (offset error).
Shaft angle error of gear box.
Mounting distance error.
Errors 1 and 2 can be corrected only by reprocessing the housing/mounting. Error 3 can be corrected by adjusting the gears in an axial direction. All three errors may be the cause of improper backlash.
19.2.1 The Offset Error of Shaft Alignment
If a gear box has an offset error, then it will produce crossed end contact, as shown in
. This error often appears asif error is in the gear tooth orientation.
19.2.2 The Shaft Angle Error of Gear Box
shows, the contact trace will move toward the toe end if the shaft angle error is positive; the contact trace will move toward the heel end if the shaft angle error is negative.
19.2.3 Mounting Distance Error
When the mounting distance of the pinion is a positive error, the contact of the pinion will move towards the tooth root, while the contact of the mating gear will move toward the top of the tooth. This is the same situation as if the pressure angle of the pinion is smaller than that of the gear. On the other hand, if the mounting distance of the pinion has a negative error, the contact of the pinion will move toward the top and that of the gear will move toward the root. This is similar to the pressure angle of the pinion being larger than that of the gear. These errors may be diminished by axial adjustment with a backing shim. The various contact patterns due to mounting distance errors are shown in
Mounting distance error will cause a change of backlash; positive error will increase backlash; and negative, decrease. Since the mounting distance error of the pinion affects the surface contact greatly, it is customary to adjust the gear rather than the pinion in its axial direction.
19.3 Surface Contact Of Worm And Worm Gear
There is no specific Japanese standard concerning worm gearing, except for some specifications regarding surface contact in JIS B 1741. Therefore, it is the general practice to test the tooth contact and backlash with a tester.
shows the ideal contact for a worm gear mesh.
, we realize that the ideal portion of contact inclines to the receding side. The approaching side has a smaller contact trace than the receding side. Because the clearance in the approaching side is larger than in the receding side, the oil film is established much easier in the approaching side. However, an excellent worm gear in conjunction with a defective gear box will decrease the level of tooth contact and the performance.
There are three major factors, besides the gear itself, which may influence the surface contact:
Shaft Angle Error.
Center Distance Error.
Mounting Distance Error of Worm Gear.
Errors number 1 and number 2 can only be corrected by remaking the housing. Error number 3 may be decreased by adjusting the worm gear along the axial direction. These three errors introduce varying degrees of backlash.
19.3.1. Shaft Angle Error
If the gear box has a shaft angle error, then it will produce crossed contact as shown in
. A helix angle error will also produce a similar crossed contact.
19.3.2 Center Distance Error
Even when exaggerated center distance errors exist, as shown in
, the results are crossed end contacts. Such errors not only cause bad contact but also greatly influence backlash. A positive center distance error causes increased backlash. A negative error will decrease backlash and may result in a tight mesh, or even make it impossible to assemble.
19.3.3 Mounting Distance Error
shows the resulting poor contact from mounting distance error of the worm gear. From the figure, we can see the contact shifts toward the worm gear tooth's edge. The direction of shift in the contact area matches the direction of worm gear mounting error. This error affects backlash, which tends to decrease as the error
increases. The error can be diminished by microadjustment of the worm gear in the axial direction.
SECTION 20: LUBRICATION OF GEARS
The purpose of lubricating gears is as follows:
Promote sliding between teeth to reduce the coefficient of friction
Limit the temperature rise caused by rolling and sliding friction.
To avoid difficulties such as tooth wear and premature failure, the correct lubricant must be chosen.
20.1 Methods Of Lubrication
There are three gear lubrication methods in general use:
Splash lubrication (oil bath method).
Forced oil circulation lubrication.
There is no single best lubricant and method. Choice depends upon tangential speed (m/s) and rotating speed (rpm). At low speed, grease lubrication is a good choice. For medium and high speeds, splash lubrication and forced circulation lubrication are more appropriate, but there are exceptions. Sometimes, for maintenance reasons, a grease lubricant is used even with high speed.
presents lubricants, methods and their applicable ranges of speed.
The following is a brief discussion of the three lubrication methods.
20.1.1 Grease Lubrication
Grease lubrication is suitable for any gear system that is open or enclosed, so long as it runs at low speed. There are three major points regarding grease:
Choosing a lubricant with suitable viscosity. A lubricant with good fluidity is especially effective in an enclosed system.
Not suitable for use under high load and continuous operation. The cooling effect of grease is not as good as lubricating oil. So it may become a problem with temperature rise under high load and continuous operating conditions.
Proper quantity of grease. There must be sufficient grease to do the job. However, too much grease can be harmful, particularly in an enclosed system. Excess grease will cause agitation, viscous drag and result in power loss.
20.1.2 Splash Lubrication
Splash lubrication is used with an enclosed system. The rotating gears splash lubricant onto the gear system and bearings. It needs at least 3 m/s tangential speed to be effective. However, splash lubrication has several problems, two of them being oil level and temperature limitation.
There will be excessive agitation loss if the oil level is too high. On the other hand, there will not be effective lubrication or ability to cool the gears if the level is too low.
shows guide lines for proper oil level. Also, the oil level during operation must be monitored, as contrasted with the static level, in that the oil level will drop when the gears are in motion. This problem may be countered by raising the static level of lubricant or installing an oil pan.
The temperature of a gear system may rise because of friction loss due to gears, bearings and lubricant agitation. Rising temperature may cause one or more of the following problems:
- Lower viscosity of lubricant.
- Accelerated degradation of lubricant.
- Deformation of housing, gears and shafts.
- Decreased backlash.
New high-performance lubricants can withstand up to 80 to 90°C. This temperature can be regarded as the limit. If the lubricant's temperature is expected to exceed this limit, cooling fins should be added to the gear box, or a cooling fan incorporated into the system.
20.1.3 Forced-Circulation Lubrication
Forced-circulation lubrication applies lubricant to the contact portion of the teeth by means of an oil pump. There are drop, spray and oil mist methods of application.
An oil pump is used to suck-up the lubricant and then directly drop it on the contact portion of the gears via a delivery pipe.
An oil pump is used to spray the lubricant directly on the contact area of the gears.
Oil mist method:
Lubricant is mixed with compressed air to form an oil mist that is sprayed against the contact region of the gears. It is especially suitable for highspeed gearing.
Oil tank, pump, filter, piping and other devices are needed in the forcedlubrication system. Therefore, it is used only for special high-speed or large gear box applications. By filtering and cooling the circulating lubricant, the right viscosity and cleanliness can be maintained. This is considered to be the best way to lubricate gears.
20.2 Gear Lubricants
An oil film must be formed at the contact surface of the teeth to minimize friction and to prevent dry metal-to-metal contact. The lubricant should have the properties listed in
20.2.1 Viscosity of Lubricant
The correct viscosity is the most important consideration in choosing a proper lubricant. The viscosity grade of industrial lubricant is regulated in JIS K 2001.
expresses ISO viscosity grade of industrial lubricants.
JIS K 2219 regulates the gear oil for industrial and automobile use.
shows the classes and viscosities for industrial gear oils.
JIS K 2220 regulates the specification of grease which is based on NLGI viscosity ranges. These are shown in
Besides JIS viscosity classifications,
contains AGMA viscosity grades and their equivalent ISO viscosity grades.
20.2.2 Selection Of Lubricant
It is practical to select a lubricant by following the catalog or technical manual of the manufacturer.
is the application guide from AGMA 250.03 "Lubrication of Industrial Enclosed Gear Drives".
is the application guide chart for worm gears
from AGMA 250.03.
expresses the reference value of viscosity of lubricant used in the equations for the strength of worm gears in JGMA 405-01.
SECTION 21: GEAR NOISE
There are several causes of noise. The noise and vibration in rotating gears, especially at high loads and high speeds, need to be addressed. Following are ways to reduce the noise. These points should be considered in the design stage of gear systems.
Use High-Precision Gears
- Reduce the pitch error, tooth profile error, runout error and lead error.
- Grind teeth to improve the accuracy as well as the surface finish.
Use Better Surface Finish on Gears
- Grinding, lapping and honing the tooth surface, or running in gears in oil for a period of time can also improve the smoothness of tooth surface and reduce the noise.
Ensure a Correct Tooth Contact
- Crowning and relieving can prevent end contact.
- Proper tooth profile modification is also effective.
- Eliminate impact on tooth surface.
Have A Proper Amount of Backlash
- A smaller backlash will help reduce pulsating transmission.
- A bigger backlash, in general, causes less problems.
Increase the Contact Ratio
- Bigger contact ratio lowers the noise. Decreasing pressure angle and/or increasing tooth depth can produce a larger contact ratio.
- Enlarging overlap ratio will reduce the noise. Because of this relationship, a helical gear is quieter than the spur gear and a spiral bevel gear is quieter than the straight bevel gear.
Use Small Gears
- Adopt smaller module gears and smaller outside diameter gears.
Use High-Rigidity Gears
- Increasing face width can give a higher rigidity that will help in reducing noise.
- Reinforce housing and shafts to increase rigidity.
Use High-Vibration-Damping Material
- Plastic gears will be quiet in light load, low speed operation.
- Cast iron gears have lower noise than steel gears.
Apply Suitable Lubrication
- Lubricate gears sufficiently.
- High-viscosity lubricant will have the tendency to reduce the noise.
Lower Load and Speed
- Lowering rpm and load as far as possible will reduce gear noise.
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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