Engineering Information

Backlash

Backlash is defined as the rotational arc clearance between a pair of mounted gears. In any gear set, some amount of backlash (clearance) is necessary to prevent damage brought about by gear tooth interference. Lack of backlash may cause noise, overloading, overheating of gears and bearings and even seizing and failure. Backlash is measured by restricting the rotation of one member, usually the pinion, and measuring the rotational movement of the other component at some reference radius. WINSMITH has historically used three inches as a reference radius, but any convenient distance is applicable, remembering that greater distances will result in more accurate measurements. Backlash is usually stated in arc degrees (minutes/seconds). The resulting arc movement measurement can be converted to degrees using the following equation:

The resulting decimal can be converted to minutes and seconds by multiplying by 60 (minutes/degree). The integer value is the arc minutes. The arc seconds are obtained by multiplying the decimal remainder again by 60.
 

When measuring the backlash in spur or helical gears, the arc movement of either the pinion or gear can be measured. The result will be identical. When measuring the backlash in worm gear units, the arc movement of the slow speed shaft (gear) is measured while restricting the rotation of the input shaft (worm). It is not correct to measure the worm arc movement while restricting the gear rotation as this result will be much greater and is not indicative of tooth clearance (backlash). Axial clearances in the high speed worm bearings will add to the arc movement of the gear and "appear’’ to be backlash. This clearance is minimal and for most applications, is of no consequence. However, when close backlash is required, bearing endplay must be considered and reduced if necessary.

Tolerance variations in the related components will affect backlash. These include housing center distance variations, gear geometry tolerances and bearing runout. These and other issues must be considered when establishing a design specification for backlash. Closer tolerances in the housing and gear geometry along with higher precision bearings combine to provide closer backlash control, enabling tighter backlash when needed but at a greater cost. Therefore, the maximum allowable backlash for the application should be specified.

Standard backlash (no special requirements) is suitable when units operate continuously in a single direction in the absence of load reversals (ie. when the torque changes direction causing separation and re-engagement of the tooth flanks). Closer backlash is recommended for applications involving frequent starting and stopping, reverse rotation or where load reversals (explained above) are present. WINSMITH reduced (minimum) backlash specifications (see below) have traditionally accommodated these applications. Here, high speed bearing endplay (axial movement) should be kept to a minimum to reduce the impact during stopping or load reversing.

For those applications requiring near zero backlash for positioning or some other unique situation, special designs which allow for backlash adjustment are usually necessary. WINSMITH has developed two different designs to accommodate near zero backlash. One concept uses eccentric bearing seats in the slow speed covers that allows for a center distance adjustment when the covers are rotated. Although very close backlash can be obtained, this method has a finite number of adjustments and is limited in its ability to approach zero back-lash. In addition, it is only suitable for units with two slow speed covers, curtailing its use. This method has generally been limited to units in the WINSMITH C-Line.

A second, more recent concept named the S-Eliminator, uses a variable lead worm where the worm thread thickness increases along its length. Backlash is reduced by moving the worm axially (via an adjustment cartridge) until the worm thread thickness closely matches the gear tooth space. This method provides infinite adjustment and can easily achieve near zero backlash. In addition, as the gear wears in, the backlash can be easily readjusted in the field using the built-in adjustment cartridge. This feature adapts well to the D-90 SE product line and can also be adapted to the C-Line. 

The Table I below summarizes the backlash specifications for the four WINSMITH offerings. Table II summarizes the general product selections for these four offerings.