ASSEMBLING BEVEL GEARS
Mounting bevel gears   Mountings  Mounting distance  Backlash  Installing the gears
Marked teeth  Locating the pinion  Locating the gear  Checking the installation
Rules for installing bevel gears
Tooth bearings: Spiral bevel gears-(diagrams)  Hypoid gears   Zerol gears  Straight bevel gears


Page 2

Mounting bevel gears

Every pair of gears represents an expenditure of much time and effort both in design and manufacture. Corresponding care is required when the gears are assembled to obtain full benefits of the accuracy and good workmanship which has been put into them. The best gears that can be made will not perform properly and may be entirely unusable, if not correctly installed. Only with correct mounting will the maximum in efficiency, wear, strength, and smoothness of operation be obtained.

Mountings

It is assumed that the gear mountings will be rigid. When mountings allow excessive deflection, there is danger that the load may be concentrated at the ends of the teeth, causing them to break. The shafts which carry the gears must be accurately located and set at the correct angle in the same plane. Moreover, the gears must be a close fit on the shaft, as looseness of fit is almost certain to introduce eccentricity.

In the mounting for spiral bevel or hypoid gears, provision must be made to lock both the gear and pinion against axial thrust in both directions, as under certain conditions either member may have an inward as well as an outward thrust.

Provisions in the housing should also be made for moving the pinion axially to obtain correct mounting distance and for moving the gear axially to secure the specified backlash.

Mounting distance

The axial position of a bevel gear or pinion in assembly is given by a dimension called the mounting distance. (See Figure 1).

Figure 1.  Mounting distance

This is the linear dimension from the axial locating surface of a given member to the intersection point of its axis with the axis of its mate. This surface is usually the back face of the pinion or gear. Frequently, it is more convenient to make measurements in assembly to the front of the pinion. In such cases, a flat reference surface is provided at the front for measuring purposes.

In the case of a hypoid gear or pinion, the mounting distance is the linear dimension from its axial locating surface to the common perpendicular from its axis to the axis of its mate.

In quantity production, it is recommended that the gears be manufactured so that the best running position is at the specified mounting distance. Any gears which are not held to exact mounting distance should be matched in sets with the mounting distance of each piece carefully etched on the outer end of the teeth, as shown in figure 4. In this way difficulties and errors in assembly will be reduced.

Backlash

Bevel Gears are cut to have a definite amount of backlash which varies according to the pitch and operating conditions. This backlash is necessary for the safe and proper running of the gears. If there is insufficient backlash, the gears will be noisy, wear excessively, and possibly score on the tooth surface or even break. The amount of backlash required is usually etched or stamped on one or both members of a pair. If no markings are given, use the values listed in the following table :

Table of normal backlash

This table gives the recommended normal backlash for gears assembled ready to run. Backlash for pitches other than those listed may be obtained by interpolation. In certain applications the backlash tolerance may have to be altered to meet specific requirements.

Normal backlash, that is, backlash in a directional perpendicular to the tooth surface, should be measured at the tightest point. In order to make this measurement, hold the pinion solidly against rotation, rigidly mount a dial indicator against a gear tooth, being sure that the indicator stem is perpendicular to the tooth surface at the extreme heel, as shown in figure 2. The normal backlash will then be shown by the indicator when the gear is turned back and forth. To obtain the backlash in the plane of rotation, figure 6, the normal backlash must be divided by the cosine of the spiral angle and the cosine of the pressure angle.

In the case of fine-pitch gears, it is often easier to check the backlash by means of a boom arrangement attached to the gear shaft. The indicator reading is taken at a radius approximately equal to the pitch radius of the gear. This method gives a direct reading of backlash in the plane rotation.

This checking of the backlash is an important step in the mounting of the gears. When the requisite backlash is not obtained, the gear should be adjusted axially until it is obtained. The axial movement of the gear necessary to effect a 0.001" change in normal backlash varies with the pressure angle approximately as follows:

Figure 2. Location of indicator when checking Backlash

top of page    home page