Technical Resources Molded vs. Cut Plastic Gears, Elements of Metric Gear Technology, Page 11
Molded vs. Cut Plastic Gears, Elements of Metric Gear Technology (Cont.)
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. Table 18-21 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.
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. Figure 18-9 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 Figure 18-10.
- Locating Ring is the element which assures the proper location of the mold on the platen with respect to the nozzle which injects the molten plastic.
- Sprue Bushing is the element which mates with the nozzle. It has a spherical or flat receptacle which accurately mates with the surface of the nozzle.
- Sprue is the channel in the sprue bushing through which the molten plastic is injected.
- Runner is the channel which distributes material to different cavities within the same mold base.
- Core Pin is the element which, by its presence, restricts the flow of plastic; hence, a hole or void will be created in the molded part.
- Ejector Sleeves 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.
- Front Side is considered the side on which the sprue bushing and the nozzle are located.
- Gate is the orifice through which the molten plastic enters the cavity.
- Vent (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.
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 (Figure 18-12). 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, 18-14 part 2, and 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, Figure 18-15 can serve as an illustration.
SECTION 19: FEATURES OF TOOTH SURFACE CONTACT
- 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 Table 19-1. The percentage in Table 19-1 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 Figure 19-1; 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.
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 Figure 19-2. This error often appears asif error is in the gear tooth orientation.
19.2.2 The Shaft Angle Error of Gear Box
As Figure 19-3 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 Figure 19-4.
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. Figure 19-5 shows the ideal contact for a worm gear mesh.
From Figure 19-5, 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.
19.3.1. Shaft Angle Error
If the gear box has a shaft angle error, then it will produce crossed contact as shown in Figure 19-6. 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 Figure 19-7, 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
Figure 19-8 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 (µ).
- Limit the temperature rise caused by rolling and sliding friction.
20.1 Methods Of Lubrication
There are three gear lubrication methods in general use:
- Grease lubrication.
- Splash lubrication (oil bath method).
- Forced oil circulation lubrication.
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.
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.
- Oil level:
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. Table 20-2 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.
- Temperature limitation:
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.
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.
- Drop method:
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.
- Spray method:
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.
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 Table 20-3.
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. Table 20-4 expresses ISO viscosity grade of industrial lubricants.
JIS K 2219 regulates the gear oil for industrial and automobile use. Table 20-5 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 Table 20-6.
Besides JIS viscosity classifications, Table 20-7 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. Table 20-8 is the application guide from AGMA 250.03 "Lubrication of Industrial Enclosed Gear Drives".
Table 20-9 is the application guide chart for worm gears from AGMA 250.03.
Table 20-10 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 Section 7:
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 References and Literature of General Interest