How to use stepper motors to solve load problems of increasing size and weight

How to use stepper motors to solve load problems of increasing size and weight

Increasing size and weight increases the inertia of the load and the torque required to move the load. Gearing is a good choice for overcoming large inertia loads because it reduces the reflected inertia on the motor by the square of the gear ratio.

Adding gear ratios also increases the amount of torque produced. Generally, the torque produced is equal to the motor torque multiplied by the gear ratio and the efficiency of the gears. However, sometimes the output torque is limited for other mechanical reasons, such as gear tooth strength, gearbox strength, or carrier strength.

Tg = Tm x GR x efficiency

There are a variety of gears available. The decision on which gear to use will come down to the torque, speed, and backlash requirements. A summary of each gear type is listed below.

In order to move as fast as possible, acceleration torque must be considered. The more torque available for acceleration, the faster it can move. When comparing gears, planetary gears have more available torque than bevel hobbed gears or spur gears. This is because planetary gears can spread the force between all the planetary gears, rather than each gear taking the full force. Therefore, a planetary gear will be able to move faster than a bevel hobbed gear or spur gear.

T: Torque applied to each planetary gear (Nm)

T': Total torque delivered (Nm)

n: number of planetary gears

k: coefficient of dispersion

Below is an example of how more available torque can allow for faster acceleration. Bevel hobbing gears and PS planetary gears are used in the example with a 60mm motor.

Maximum Torque

60 mm TH: 35 lb-in

60 mm PS: 177 lb-in

More available torque means we can accelerate the motor harder, reducing the time it takes to move.

How to address the increased load in size and weight

disk

20 inch diameter

20 lbs

Motion curve

360° in 1.5 seconds

One direction of movement (CW)

Safety factors

Minimum 1.5

System requirements

Single phase 115 VAC

Requires motor and driver

In order to keep the load inertia to rotor inertia ratio below 10:1, a gear ratio of at least 40.8 must be used. This eliminates tapered hobbing and spur gears from possible solutions. Since the motion is only clockwise, backlash is not an issue, so we will look at a 50:1 PS planetary gear.

The maximum speed of a 50:1 planetary gear is 60 RPM, so this is still an option.

The load torque calculated above is based on a coefficient of friction of 0.05, a distance of 1 inch from the center of the table to the support structure, and an overall system efficiency of 90%.

The required torque is 1092 oz-in or 68.25 lb-in.