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Selecting gearboxes for servo motors

Selecting gearboxes for servo motors

Servo gearboxes differ from standard industrial gearboxes in that they are designed to operate with higher peak input speeds together with a wide operating range. Other factors such as low inertia, backlash and exact numeric ratios can also be important. A common choice to be paired up with servo motors is the planetary design which has compact dimensions and high performance. However planetary is not the only option and other factors such as the mounting arrangement can lead to selection of alternatives. They key decision factors here are the required torques, gear ratio, level of backlash that is acceptable and the mounting configuration.

Many machine designers look for servo gearboxes with low backlash. Generally the catalogue values for backlash are listed for a torque of 2% of the rated value measured in arcmins at the output shaft. Zero backlash is not a practical option as the lack of clearance leads to elevated running temperatures, but figures as low as 1 arcmin are possible. This is only 1/60th of a degree.  More commonly low backlash gearboxes are widely available at 3 to 5 arcmins.

Of course low backlash comes with a cost. Not only do purchase costs rise, but noise levels can also be higher. Gearboxes will run hotter and the higher temperatures will degrade the lubricant in the long term so reducing the lifetime. Therefore low backlash should be specified where it is needed for reversing drives where positioning accuracy is needed. Drives that always run in one direction can operate with medium backlash gearboxes unless external forces can cause movement away from the stopped position.

Servo gearboxes are designed for cyclic duties and the profile of the cycle plus the frequency will influence the selected size of the gearbox. For example there will be catalogue values for acceleration torque, rated torque and emergency stop torque. The acceleration torque required is simple to calculate and must be below the catalogue limit, if necessary with factors applied for high frequency of operation and high ambient temperature. The rated torque is the limit for the average required torque over a period of time. The emergency stop torque is a limiting value that applies for 1000 stops over the life of the gearbox.

Similarly there will be two catalogue figures for permissible speeds. The maximum input speed is a high value, typically 4000-6000r/min depending on gearbox size, that must not be exceeded. However the more important value for selection is the maximum permissible average speed which will be substantially lower and depends on the load profile.

Different types of servo gearboxes offer a choice of mounting configurations. Where there is a male shaft, this can have a key and keyway but is often supplied without a keyway. Slight backlash is virtually impossible to avoid in keyways which can lead to fretting wear on highly dynamic drives. Therefore a plain shaft to be fitted with a friction locking bush is often preferred.  Similary hollow shafts can be supplied with or without keyways. Where the option without keyway is chosen, the hollow shaft is extended to one side beyond the gearbox casing and a shrink disc type of locking assembly makes the connection.

Servo gearboxes are not standardised products and there are a number of variants available. For example, servo planetary gearboxes with flanged outputs instead of shafts are available. Not only do these give shorter build lengths, but also bigger bearings in the casing increase radial load capacity. The Vogel SKPR is a variant where an additional planetary stage is added to their hypoid range making ratios up to 100 and torques to 1000Nm possible. Servo gearboxes can also be supplied with ATEX certification. 

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