What is a ball detent torque limiter?
A ball detent torque limiter is essentially a mechanical circuit breaker designed to release the load when it senses an overload condition. This type of device is not to be confused with a shear pin which is basically a mechanical fuse designed to break the circuit and be replaced. A ball detent torque limiter is also not to be confused with a slip clutch which will “slip” to maintain a constant peak torque value when an overload is sensed.
What are the advantages of ball detent torque limiters?
The torque limiter’s main advantages over shear pin technology are maintenance time and disengagement accuracy. A torque limiter can disengage at +/-5% versus a shear pin at +/-20% (depending on materials of construction and design). This is very important in designs which must protect sensitive equipment. Ball detent torque limiter designs either automatically re-engage when the over-torque condition is corrected or can be manually re-engaged. This can save a lot of time in maintenance down-time while replacing a broken shear pin as well as the cost of purchasing a new shear pin each time. Depending on the application, it may be preferable to use a ball detent torque limiter over a slip clutch. Because the torque limiter completely releases the load rather than maintaining a peak torque, it can mitigate the likelihood that damage occurs.
How do you size a mechanical torque limiter?
The proper way to size a mechanical torque limiter is to know the exact intended disengagement value. This value is set at a point below the torque value at which possible damage to the system could occur. It is also very important to know the normal operating torque of the system. This way, a torque range at which the limiter must disengage can be determined. If the range is known and a disengagement point is unknown, at the very least, an adjustable torque limiter can be implemented. Adjustments can be made using trial and error beginning at a value just above operating torque and adjusting up until nuisance tripping stops.
If no torque values are known, measures should be taken to find them such as measuring manually with gauges or back calculating from maximum chain pulls or allowable force on bearings, etc. Torque can also be calculated using Horse Power and RPM. The formula: Torque=5252*HP/RPM is very useful. As we can see, if HP is constant, Torque will decrease as the speed increases. This is very important to note in test stand applications where a large electric motor is used. Many test stands run at 20,000 + RPM where a large motor is producing a seemingly unusual low amount of torque. Conversely, running at low RPM can produce higher than expected torque values.
If an operating torque can be determined, then it is generally best practice to size a torque limiter with a range beginning at the operating torque and ranging up to 2-3 times above that value. Having a torque limiter with a set range a good amount above the operating value is often the best way to design especially if inductive motors are to start a load into motion. Other considerations are whether the system is to be started under load or not. Often times, in these cases, a torque limiter may have an adjustment range many times above the normal operating range. This is often true in rock crusher and wood mill applications. More detailed information including sizing formulas is available in the R+W SK Series catalog. Lastly, but most importantly, always keep in mind the safety of any personnel who will be around working around the drive line.