Even if the gear has high accuracy, if the gear is not mounted properly it is not possible to avoid problems regarding bad tooth contact noise, wear, and breakage.
8.1 Accuracy of Center Distance
Error in the center distance influences the backlash of the gear mesh. If the center distance value increases, the backlash value is increased. As the result, gear teeth can not mesh deeply enough each other, and the contact ratio decreases. If the center distance value decreases, the backlash value also decreases. Gears may not rotate if the backlash decreases too much.
Table 8.1 shows the center distance tolerance of spur gears and helical gears. The tolerance values in this table are quoted from JGMA1101-01(2000), and are applicable for involute spur and helical gears, made of iron and steel.
Table 8.1 Center Distance Tolerance of Spur Gears and Helical Gears ± fa
Fig. 8.1 Shaft Parallelism Error and Shaft Offset Error
8.2 Axial Parallelism
The accuracy of two parallel axis is composed with parallelism error and shaft offset error. These errors influence the tooth contact in the tooth trace direction. It may result in bad tooth contact occurring at the tip of tooth width. Increase of the error involves decreasing the backlash or causing of noise by tooth breakage.
Table 8.2 and 8.3 shows Shaft parallelism error and offset error tolerance of spur and helical gears, where data was selected from JGMA1102-01(2000).
Table 8.2 Allowable in-plane deviation with respect to parallelism of axes per facewidth fx / Unit : µm
Table 8.3 Allowable out-of-plane deviation with respect to parallelism of axes per facewidth fy / Unit : µm
8.3 Features of Tooth Contact
Tooth 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 end relief. - 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).
The features above are all related to the production of gears/ gearboxes, or to the accuracy of modification. In spite of efforts of prevention, tooth contact problems still may occur at the final inspection before mounting, in some cases. If this happens, tooth contact of spur and helical gears can reasonably be controlled by shifting the gear in axial direction.
Proper tooth contact is one of the elements in providing gear accuracy and very important for bevel gear and worm gear pairs. Compared to spur or helical gears, it is more difficult to inspect gear accuracy of bevel gears and worm gear pairs. Consequently, final inspection of bevel and worm mesh tooth contact in assembly provides a quality criteria for control. JGMA1002-01 (2003) classifies tooth contact into three levels, A, B, C, as presented in Table 8.4.
Table 8.4 Levels of tooth contact
The percentage in Table 8.4 considers only the effective width and height of teeth.
8.3.1 Tooth Contact of a Bevel Gear
It is important to check the tooth 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 8.2; and, as load is increased to full load, contact shifts to the central part of the tooth width.
Fig. 8.2 Central toe contact
Even when a gear is ideally manufactured, it may reveal poor tooth 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 gearbox
- 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.
(1) The Offset Error of Shaft Alignment
If a gearbox has an offset error, then it will produce crossed contact, as shown in Figure 8.3. This error often appears as if error is in the gear tooth orientation.
Fig. 8.3 Poor tooth contact due to offset error of shafts
(2) The Shaft Angle Error of Gear Box
As Figure 8.4 shows, the tooth contact will move toward the toe end if the shaft angle error is positive; the tooth contact will move toward the heel end if the shaft angle error is negative
Fig. 8.4 Poor tooth contact due to shaft angle error
(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 8.5.
Fig .8.5 Poor tooth contact due to error in mounting distance
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 tooth contact greatly, it is customary to adjust the gear rather than the pinion in its axial direction.
8.3.2 Tooth Contact of a Worm Gear Pair
There is no specific Japanese standard concerning worm gearing, except for some specifications regarding tooth contact in JGMA1002-01 (2003). Therefore, it is the general practice to test the tooth contact and backlash with a tester. Figure 8.6 shows the ideal contact for a worm mesh.
Fig. 8.6 Ideal tooth contact of worm gear pair
From Figure 8.6, we realize that the ideal portion of contact inclines to the receding side.
Because the clearance in the approaching side is larger than on the receding side, the oil film is established much easier in the approaching side. However, an excellent worm wheel in conjunction with a defective gearbox will decrease the level of tooth contact and the performance. There are three major factors, besides the gear itself, which may influence the tooth contact :
- Shaft Angle Error.
- Center Distance Error.
- Locating Distance Error of Worm Wheel.
Errors 1 and 2 can only be corrected by remaking the housing. Error 3 may be decreased by adjusting the worm wheel along the axial direction. These three errors introduce varying degrees of backlash.
(1) Shaft Angle Error
If the gear box has a shaft angle error, then it will produce crossed contact as shown in Figure 8.7. A helix angle error will also produce a similar crossed contact.
Fig. 8.7 Poor tooth contact due to shaft angle error
(2) Center Distance Error
Even when exaggerated center distance errors exist, as shown in Figure 8.8, the results are crossed contact. 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.
Fig. 8.8 Poor tooth contact due to center distance error
(3) Locating Distance Error
Figure 8.9 shows the resulting poor contact from locating distance error of the worm wheel. From the figure, we can see the contact shifts toward the worm wheel tooth’s edge. The direction of shift in the contact area matches the direction of worm wheel locating error. This error affects backlash, which tends to decrease as the error increases. The error can be diminished by micro adjustment of the worm wheel in the axial direction.
Fig.8.9 Poor tooth contact due to mounting distance error