Counteracting gravity: General Industry Coverage

The vertical linear axes on machine tools are fundamentally different than those that operate parallel to the ground. This is because the actuators on the vertical axes have to overcome the effects of gravity. In addition to the forces required to accelerate the axes, the weight of the moving elements imposes a constant load that servomotors or other actuators must carry to hold the axes in position (Figure 1).

Courtesy of All images courtesy S. Smith

Figure 1. The servomotor carries the weight of vertical axis components. The vertical moving elements are red, the servomotor is green, and the ballscrew is yellow. The force supplied by servomotor F carries the weight of the moving elements, which equals m×g, where m is the mass, and g is the gravitational acceleration.

On small machine tools, that load may be small enough to ignore, but on larger tools, the power required to hold against the weight is significant. Power costs money, and power is eventually transformed into heat that is conducted into the machine tool structure.

In many machines, the weight of the vertical moving components is enough to drive the ballscrew backward if the servomotor loses power. Therefore, each vertical axis has a brake, which requires power to release. If power to the vertical actuator is turned off or lost, the brake is engaged before the axis can drop.

Several methods are commonly employed to reduce the weight load on the vertical actuator. An obvious solution is a counterweight (Figure 2). In the figure, a mass equal to the mass of the original vertical components has been added. The two masses are connected by a cable-pulley system to balance them, and the servomotor only has to supply the force needed for acceleration.