ARCILLAS

Some design applications call for severe limits on the maximum backlash at the point of loosest mesh. These limits cannot be met using the foregoing design procedure even if the tolerances are made as small as practical. Some applications may even require zero backlash. To meet such conditions, the designer can use some special methods of back- lash control, of which a number of examples using spur and helical gears are described here.

7.5.1 Selective assembly

Selective assembly is one way of meeting maximum backlash requirements without further reducing center distance and tooth thickness tolerances. Usually one gear in the gear set is manufactured in a range of sizes and, at assembly, the proper size is selected to give the desired minimum backlash at the point of tightest mesh. With this method, it is still necessary to limit the total composite tolerances to achieve small enough backlash at the point of loosest mesh.

7.5.2 Center distance control

Controlling backlash by change in center distance can be accomplished either by adjustment or by a spring forcing the gears together. In either case, the center distance tolerance and the tooth thickness tolerances no longer influence the backlash. However, the total composite tolerances are still important.

7.5.2.1 Adjustment

In this method, the bearings supporting one of the gears can be moved to obtain the desired minimum backlash at the point of tightest mesh. One such arrangement is shown in figure 46. Another possible arrangement uses eccentric bushings in fixed housings. After adjustment, the bushings must be clamped tight enough to resist the forces developed by the applied loads. Adjustment has a relative disadvantage. Due to the total composite variations of the two gears, the backlash for other rotated positions of the gears will be greater than the adjusted tightest mesh value.

Figure 46 -- Adjustable center distance gearing

7.5.2.2 Spring loading

In this method, the bearings supporting one of the gears are free to move under the influence of spring forces so as to tighten the gear center distance. Except for this movement, the bearings are fully constrained (see figure 47). This results in zero backlash for all rotational positions of the gears. The spring forces must be kept greater than the opposing bearing forces developed by the gear loads and other applied loads. This condition causes wedging of the gear teeth and increased gear and bearing friction. Because of the total composite variations in the gears, there is vibration of the spring loaded bearings and further friction losses. This method of backlash control is not suitable for applications with high loads or high speeds.

Figure 47 -- Spring loaded center distance gearing

7.5.3 Split gearing

In split gearing, one of the gears in the mating pair is constructed of two half--gears side--by--side (see figures 48 and 49). Only one half is directly fastened to its shaft. Rotation of one half relative to the other has the effect of changing their combined tooth thickness and thereby controls the backlash. This relative rotation may be used in the form of an adjustment during assembly or it may be left as a continuing action under spring loading. In either case, the center distance tolerance and the tooth thickness tolerances no longer influence the back- lash. Split gearing has the disadvantage of its more complex construction and the added space it needs and the added inertia it introduces. If the gears must transmit the same load in both directions, the split design means the doubling or tripling of the overall size of the split gear and a corresponding increase in the face width of the one--piece mating gear.

7.5.3.1 Adjustable

In this method, the two half gears are adjusted and clamped, as in figure 48, to give the desired minimum backlash at the point of tightest mesh with the mating gear. The clamping must be tight enough to resist the operating gear loads. The total composite variations of all the gears will be reflected in some increase in backlash during their rotational cycle.

Figure 48 -- Adjustable split gearing 7.5.3.2 Spring loaded

The spring loaded design gives zero backlash for all rotational positions of the gears. A common construction for such gearing is shown in figure 49.

The spring forces must be kept great enough to withstand the transmitted gear loads in the direction that opposes the spring. If the transmitted gear loads are the same in both directions of rotation, the resulting total gear tooth contact faces are in excess of three times those for simple gears. The combined total composite variations of the gears require some relative motion of the split gear sections in order to keep the zero backlash condition. These extra loads and relative motion all add to the friction losses and make this method of backlash control not suitable for applications other than those with light loads and low speeds.

Figure 49 -- Spring loaded split gearing

7.5.4 Composite gearing with elastic element

One of the gears of the mating pair is constructed with an elastic element inserted. This type of gearing consists of a metallic gear with a plastic element running through the central portion of all of the teeth. See figure 50.

When this gear is assembled with its mate of conventional design, the metallic parts of the teeth will have normal backlash. However, the oversize plastic section will take up the backlash for all but the heaviest loads.

This type will not transmit angular motion accu- rately, except under light loads, since the plastic tends to center the composite gear in the center of the mating gear tooth space. This centering varies with the amount of load applied. Consideration should be given to the increased bearing loads and friction resulting from the contact of the deformed plastic.

Figure 50 -- Composite gearing with elastic element 7.5.5 Tapered gearing

It is possible to manufacture a spur or helical gear with its tooth thickness tapering slightly from one side of its face width to the other. If two such gears are made with matching tapers and assembled so that the tapered teeth fit each other, as in figure 51, the gears will run together just as well as gears with uniform tooth thickness. With

such tapered gears, it is possible to adjust their relative axial position so as to control backlash.

Figure 51 -- Tapered gearing

The use of spring loading as a means of continually adjusting the tapered gears for zero backlash is not recommended. If a small taper angle is used, there may be wedging of the teeth and increased friction.

7.5.6 Preload

The effect of backlash in a drive train can be controlled by keeping the gear teeth contacting on

only one side of each tooth. This may be done by preloading the gear train with a spring or weight acting on the last driven gear, as in figure 52. Such preloading is possible only if the total rotation is limited to the range of the spring or weight arrangement. Unlike some other spring--loaded methods, this one does not cause wedging of the gear teeth. However, it does still add to the system friction and to the load on the gears.

Figure 52 -- Spring preloaded gearing (for limited rotation)

7.5.7 Dual path

It is possible to achieve the results of the preload method in a continuously rotating gear train by the use of the dual path design. In this design, there are two similar gear trains, a primary train to transmit the operating loads and a secondary train to

eliminate the backlash. Both are driven by the same pinion. Their final gears are both on the output shaft with the primary final gear rigidly connected and secondary gear spring connected, as shown in figure 53. As in the preload method, there is no wedging of the gear teeth but there is still the added friction and added gear loading.

Figure 53 -- Dual path spring loaded gearing 7.5.8 Contra--rotating inputs

The force necessary to keep the driving gear teeth in contact at only one side can be introduced by means of a second driving input, as shown in figure 54. This second input applies a torque opposite to, but smaller than, the torque of the main driving input. With the proper controls, this opposing torque can be adjusted to the level needed at each point in the driving cycle. This avoids the high friction and gear forces typical of spring--loading designs.

Output

Rotation Rotation Rotation

Contra--Rotating Drive Torque (Resisting) Torque Input

Figure 54 -- Contra--rotating input gearing

8 Gear drawings and specifications

8.1 General

The gear drawing should clearly depict the end product configuration and quality level without actually describing manufacturing methods. The drawing may become part of a contract between gear manufacturer and buyer. Therefore, no design detail essential to the operation of the gears should be omitted or assumed. In fairness to both parties, the drawing should be specific and complete. See ANSI Y14.6, Geometric Dimensions and

Tolerances.