Precision Gearhead Application Considerations

In selecting a gearhead, one must be mindful that gearhead selection will impact more than just the output speed and torque level at the output shaft. Some of the considerations to keep in mind should include:

Backlash

Backlash is a characteristic of gearhead and gear train construction that allows bidirectional shaft play. It can be caused by generous tolerances in gear design, tooth wear over time, slight machining errors in the gear cutting process, etc. It is measured at the output shaft of the gearbox and can vary typically from 1-7 degrees. Backlash is load dependent and will increase as the load increases. Backlash can cause significant error in a positioning system and should be compensated for. Typically, shaft encoders are mounted on the motor shaft and not the output gear shaft of small DC gearmotors. This means that the motor armature position can be different that the expected position of the output gear shaft by the level of backlash in the gear. 3 degrees at the output shaft of a gearhead could mean hundreds of encoder pulse at the motor depending on the resolution of the encoder and the ratio of the gearbox. For example, if you are using a 512 pulse shaft encoder and you have a gearhead ratio of 43:1, 3 degrees of backlash at the gearhead output shaft could mean up to 183 encoder pulses of error systematically.

Backlash can be eliminated in 1 direction by placing load tension on the shaft before initiating a move. For more dynamic bidirectional applications backlash can be compensated for electronically by using an external absolute encoder for comparison to the shaft encoder. The motion control electronics can then be programmed to correct position error.

Bearing Choice

Ball bearings are typically specified in applications where high radial and axial shaft loads are present. Be advised, however, that using ball bearings can increase audible noise in some cases. Please refer to the gearhead datasheets for shaft loading specifications. Sintered bearings are available for lower torque applications characterized by lower radial shaft loading and constant load characteristics.

Care should be used when press fitting components to a gearhead output shaft. We recommend not exceeding the press fit force ratings specified in the gearhead datasheets. This can damage the bearings and the internal gears themselves. In some cases, gearhead shaft bearings (ball bearings only) are preloaded with a small wave washer under the retaining ring on the bearing. Exceeding the press fit force specification on the datasheet can damage this wave washer and negate the preload on the bearing. This will affect the performance of the bearing and should always be avoided.

Lubrication

The gear and bearing lubrication can be a defining factor in gearhead performance. Gearhead bearing systems and gear trains are lubricated for life. Re-lubrication is not needed, and is not recommended. The use of non-approved lubricants on or around the gearheads or motors can negatively impact their function and life expectancy. The standard lubricants for reduction gears are formulated to provide optimum life performance with minimum current consumption at no-load condition.

Input Speed and Direction of Rotation

The input speed specification on the gearhead datasheets refers to the input speed recommended in order to maximize gearbox life. This specification is not intended to limit the gearheads to input speeds below the specification. It can be considered to be a safe mean value for operation. Your application may not require the maximum lifetime performance of the gearhead and this input speed specification may be safely exceeded depending on the performance requirements. Contact your MICROMO applications engineer for assistance if you have any questions on the gearhead input speed.

All gearheads offered by MICROMO are reversible. In the datasheets you may see an equal or not equal symbol. Don't let this confuse you. This simply means that when positive voltage is applied to the positive terminal of the motor and negative to the negative terminal that the output shaft of the gearhead, depending on the ratio, is equal to the direction of rotation of the motor or is not equal to the direction of rotation of the motor. If you have any question on any of the specifications in our datasheets, don't hesitate to contact one of our application engineers for assistance.

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Tuesday, April 08, 2008 12:12:00 PM Categories: Gearhead Tutorials

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