Mechanical torque limiters still make sense

Electrical overload protection has been used since the days of the first industrial electric motors. Back then it simply protected motors against thermal overloads. Today, however, protection devices monitor many parameters - such as current, voltage, force, torque, rotation frequency, position, temperature, and pressure - and initiate corrective measures based on these inputs.

Further, with improvements in today's drives, such as lighter and more powerful motors with more-sophisticated electronic controls, response times for corrective actions are shorter than ever. So it might seem that mechanical torque limiters are no longer necessary to protect a machine.

Collision basics

In a perfect world, overload protection would be unnecessary. But in the real world jams and crashes can never be completely eliminated. They can damage both product and equipment, resulting in expensive downtime and repairs. Causes include operator and programming errors, defective and improperly installed sensors, and external mechanical influences. Therefore some means of protection is necessary, whether mechanical, electrical, or a combination of both.

Electrical and electronic protection typically fall into two main categories: sensors and controls. Sensing devices, such as current sensors, monitor one particular aspect of the drive and provide an output or alarm when that function deviates from preset norms for a preset time period. Controls continuously monitor the machine, comparing for example the difference between actual and theoretical positions of various components. When a sensor or control detects a problem, corrective action can include stopping and reversing the drive, engaging a brake, or simply shutting down the machine.

Consider a motor and control designed to stop and reverse the drive in the event of a collision. While the machine is being stopped, braking torque M_b equals the collision torque M_k minus the drive torque M_a, or

Drive reactions have three separate phases. In Phase One, the drive is still in normal operating mode, and M_a < M_m, the positive maximum drive torque. As collision torque increases, the control compensates for the load by increasing torque in the collision direction. M_k = M_a and the resulting braking torque, M_b1 = 0.

In Phase Two, collision torque continues to increase, exceeding the maximum drive torque, M_k > M_a. The resulting difference between collision and drive torques results in a braking torque, M_b2 > 0. This begins to slow the system, although drive torque still acts in the collision direction. At this point the control recognizes the collision and initiates corrective action, in this case stopping and reversing the drive.

The duration of Phase Two depends on the time required for the control to recognize the collision plus the additional time required to react, stop, and reverse the drive. This is when damage can occur, as the drive is still supplying energy to the system in an attempt to maintain speed.

In Phase Three, drive torque acts in the braking direction and equals the negative maximum drive torque, M_a = -M_m. This produces maximum braking torque, M_b3>> 0, ideally minimizing damage and bringing the system to rest. Of course, in a control scheme where the only corrective action is simply shutting down the drive, additional damage can occur in this phase as the braking torque only equals the collision torque, M_b = M_k

Preventing damage

If, in this example, the increase in collision torque is slow and gradual (that is, a "soft" collision) and does not damage mechanical drive components such as ball screws and gearboxes, then the controls can provide adequate overload protection. This can be seen in the graph showing reactions of a mechanical overload clutch as torque increases at varying rates from 20 to 5,000 N-m/sec.

When torque increases relatively slowly, there is enough time for electrical/electronic devices to detect problems and initiate corrective actions prior to reaching the clutch's set torque.

If, however, due to the speed or mass of rotating components the collision torque rises quickly (a "hard" collision), the electronic control cannot react before significant damage occurs.

The second graph again shows reactions of a mechanical overload clutch as torque increases at varying rates, but now from 5,000 to 120,000 N-m/sec. At these higher rates of increase there is virtually no time for an electrical/electronic device to recognize the problem and begin corrective action before the torque reaches the setpoint of the clutch. In this situation, only a mechanical overload device can react quickly enough to prevent damage.

The last chart shows an actual example of a machine crash with and without mechanical overload protection. The drive in this case had a maximum torque of 28 N-m and a torque limiter adjusted to a setting of 40 N-m. Without mechanical overload protection, a maximum torque of almost 200 N-m occurs only 7.5 msec after the crash. With mechanical overload protection, although torque exceeds the 40-N-m setting due to inertia downstream of the torque limiter, it is still significantly less than maximum torque in the nonmechanical system.

The advantage of mechanical overload devices is their ability to physically disconnect drives and driven components almost instantaneously, in this case in 2.5 msec, preventing possible damage from the accumulated rotational energy upstream of the torque limiter. Even with the fastest motor and most sophisticated controls, the duration of Phase-Two conditions will be at least 5 to 10 msec, plus another 20 to 30 msec before the system comes to rest in Phase Three. Typically this is long after the system has experienced the most damaging torques.

Thus, mechanical torque limiters still make sense, but not necessarily for every application. As with the design of the drive itself, engineers should consider numerous factors before determining the best means of protection. However, relying only on electrical/electronic devices will most certainly result in equipment and machinery that is inadequately protected and prone to overload-induced damage and downtime.