What you need to know about mechanical torque limiters

Topics of discussion:
• Electronic limiters
• Mechanical limiters
• Braking torque
• Collision torque

Electronic overload protection has long been employed on industrial electric motors. In some cases, this internal protection may serve as a viable torque limiting mechanism. Many applications, however, require something more: the protection of a conventional mechanical torque limiter. Anything less will leave machinery vulnerable to overload damage and downtime. Choosing between electronic and mechanical torque limiting requires an understanding of both types of protection and where each fits best.

Electronic devices typically fall into two categories: sensors and controls. Sensors track one aspect of a drive function and provide an output when that function deviates from pre-set norms for a pre-set time period. Controls, on the other hand, continuously monitor machine functions. For example, the difference between the theoretical and actual position of various components might be compared. When either of these systems detect a problem, corrective action can be as simple as shutting down the drive to engaging a brake, or as complex as stopping and reversing the motor.

Originally electronic overload protection only prevented thermal damage. Today it is possible to monitor parameters ranging from current/ voltage, force/torque, rotational frequency, position, temperature, and pressure. Also, with lighter and more powerful motors, more sophisticated controls, and other improvements in drive systems, corrective measures initiated (based on these inputs) are made more rapidly.

Conventional mechanical torque limiters, by contrast, completely disconnect drive and driven components when an overload occurs. The most common types are shear pins, slip clutches, and balldetent torque limiters.

Other types of mechanical torque-limiting devices rely on springs (with special negative rate characteristics) that work in unison with a torque transmission system of balls interfacing two indents. In this setup, preset torque is kept within an acceptable setting tolerance and ensures that even under highly dynamic drive conditions, the clutch disengages during overload.

The design of mechanical torque limiters has also improved significantly over the years. From the basic shear-pin or slip-clutch to state-of-the-art ball-detent clutches that provide backlash free torque transmission, internal splines or keyways subject to fretting and premature wear are eliminated. The newest mechanical torque limiters also utilize special springs with negative spring rates, where increased deflection produces less end force. This eliminates false trips and “breathing”, extending service life while providing much improved accuracy and repeatability.

An example

A motor control scheme designed to stop and reverse the drive when a collision occurs typically exhibits a three phases response before the machine comes to rest, during which

M_b = M_k - M_a

Where M_b = braking torque

M_k = collision torque

M_a = drive torque

In phase one, the drive is still in its normal operating mode, with M_a less than +M_max (where +M_maxis positive maximum drive torque.) As collision torque increases, controls compensate for this load by increasing drive torque in the collision direction. With M_k equal to M_a, the resulting braking torque M_b1 equals zero.

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In phase two, collision torque continues increasing, finally exceeding maximum drive torque,so that M_k is greater than M_a. The resulting difference between the collision and drive torque results in a braking torque M_b2 greater than zero. This begins to slow the system, although the drive torque is still acting in the collision direction. It is at this point that controls recognize a collision has occurred and initiate correctiveaction. In the example considered here, the drive is stopped and reversed. The duration of phase two is dependent on the time required for the control to recognize the collision plus the additional time to react, stop, and reverse the drive. It is in this phase that damage can occur, as the drive is still negatively affecting the torque path by supplying additional energy to the system in an attempt to maintain speed.

In phase three, the drive torque now acts in the braking direction and is equal to the negative maximum drive torque — in other words, M_a = -M_max. It is here that maximum braking torque is achieved so that M_b3 is much greater than zero; ideally this minimizes any damage and brings the system to a stop. Obviously, in a control scheme where corrective action consists of only drive-shutdown, additional damage can occur in this phase as well, as braking torque will only be equal to the collision torque — M_b = M_k.

If, in this example, the increase in collision torque is slow and gradual then controls can provide adequate overload protection. This is called a soft collision, where collision torque does not reach a point where mechanical drive components, such as ball screws or gearboxes, can be damaged. At slower rates of increasing collision torque, there is adequate time prior to reaching the set torque of a clutch for an electronic device to detect a problem and initiate the necessary corrective action.

However, if collision torque rises quickly due to faster equipment speed and heavier rotating component masses, as in a hard collision, controls cannot react before there is significant damage. At these higher rates of increase, there is virtually no time for an electronic device to recognize that there is a problem and then begin corrective action before torque reaches the set point of a clutch. In this situation, only a mechanical overload device can react quickly enough to prevent damage.

Comparing real-life machine crash examples — with and without mechanical overload protection — shows the dramatic difference between resulting torques. Assume a drive has a maximum torque of 28 Nm and the torque limiter is adjusted to a torque setting of 40 Nm. Without mechanical overload protection, a maximum torque of almost 200 Nm occurs only 7.5 msec after a crash. With mechanical overload protection, although the torque does exceed the torque setting of 40 Nm due to the inertia present downstream of the torque limiter, it is still limited to less than half of that without mechanical overload protection.

This is because the advantage of a mechanical overload device is its ability to physically disconnect the drive from the driven components almost instantaneously (in this case in 2.5 msec), thereby 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 is a minimum of 5 to 10 msec in a system with a control scheme designed to stop and reverse the drive during a collision. Then it can be another 30 to even 60 msec before the system comes to rest in phase three — long after the most damaging torques have occurred.

Inevitable crashes

When considering electronic and mechanical overload devices it must be understood that jams and crashes can never be completely eliminated. They can be caused by operator and programming errors, defective or improperly installed sensors, and other external mechanical influences. So obviously some means of protection is a necessity and, even with servomotors and controls becoming more responsive and dynamic, electronic limiters can’t beat the near-instantaneous reaction of mechanical clutches. Research on the stopping times of the new range of low inertia motors from the leading manufacturers illustrates this. From maximum speed, even the fastest motors can take around 30 msec to come to a complete stop, while the most damaging torques typically occur only 10 to 20msec after a crash. Only a state of the art, mechanical torque limiting clutch can truly provide the protection required and, even though overloads are inevitable, fortunately the damage, downtime, and expensive repairs they cause is not.