Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 3 Direct Power Transfer Devices 125
ONE TIME USE TORQUE LIMITING
In some cases, the torque limit can be set very high, beyond the practical
limit of a torque limiter, or the device that is being protected needs
only a one-time protection from damage. In this case, a device called a
shear pin is used. In mobile robots, particularly in autonomous robots, it
will be found that a torque limiter is the better choice, even if a large one
is required to handle the torque. With careful control of motor power,
both accelerating and braking, even torque limiters can be left out of
most designs.
Torque limiters should be considered as protective devices for motors
and gearboxes and are not designed to fail very often. They don’t often
turn up in the drive system of mobile robots, because the slow moving
robot rarely generates an overload condition. They do find a place in
manipulators to prevent damage to joints if the manipulator gets overloaded.
If a torque limiter is used in the joint of a manipulator, the joint
must have a proprioceptive sensor that senses the angle or extension of
the joint so that the microprocessor has that information after the joint
has slipped. Figure 3-54 shows a basic shear pin torque limiter.
Figure 3-54 A shear pin is a
simple and reliable torque limiter.
However, after an overload,
removing the sheared pin stubs
and replacing them with a new
pin can be time consuming. Be
sure that spare shear pins are
available in a convenient location.