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Operator safety and protection of machines

Operator safety and protection of machines
In this artice, the experts at Lenze provide a comparison of mechanical and electronic torque limitation methods.

In this changing world some people may advise that mechanical torque limiters are old-fashioned and unnecessary, and that electronic monitoring can meet the needs of machine protection and operator safety. What is actually needed is a device to prevent damage from accidental collisions that can lead to expensive downtime. Also the increasing emphasis on operator safety as EN ISO 13849 approaches demands a higher level of protection than was acceptable before. In these cases the humble mechanical torque limiter remains the optimal answer because it disconnects the drive from the driven parts in a very short time. 

Where there is a slow increase in load, for example due to damaged bearings or dirt contamination, electronic sensing can work well. There is time to stop the drive and the signal can be used as a preventative alarm. On the other hand where the load increase is sudden, for example due to a mis-feed or mechanical breakage, only a mechanical torque limiter can give protection. By introducing a disconnection between the motor and the driven load, the torque limiter takes much of the inertial energy out of the drive train. Indeed where the limiter can be positioned at low speed close to the output, practically all the kinetic energy can be disconnected resulting in a greatly reduced possibility of damage or injury. 

Disconnection is not instantaneous; depending on the design and size of limiter it can take from single figures to tens of milliseconds. However, electronic systems cannot match this performance as they have to sense the overload and then apply a braking torque to bring the drive train to a stop. Therefore when it comes to achieving maximum protection and safety, mechanical torque limiters remain the only real answer. So what are the mechanical options?

Simple friction torque limiters drive using spring loaded rubbing faces that connect to a driven wheel, often a plate wheel. These friction torque limiters are low cost, easy to adjust steplessly, and react very quickly to shock loads. They do not suit operation in oily environments, or prolonged slippage which introduces wear and the need for readjustment. Therefore they are best used at low speed, positioned close to the output. They are available in a wide range of rated torques from 1 to 23000Nm. 

A higher consistency of release torque and the ability for operation in oily and dirty environments is met by stepping up to a ball or roller design of torque limiter. Both work by spring pressure, usually from disc type springs, that push together two plates. The plates have recesses for hardened balls or rollers and when an overload occurs the balls/rollers rise up out of their recesses and run in a track around the plates. Disconnection is not instantaneous as the springs have to be compressed when the plates move apart. Ball designs require less separation and can be more sensitive whilst roller designs are useful to avoid nuisance tripping on small torque variations, also to transmit higher torques. The release action of ball and roller types of torque limiters gives an axial movement that can be detected by a micro switch or proximity switch, allowing the drive to be turned off. 

Possible variants
Many variants are available based on the ball and roller principle. By positioning the rollers asymmetrically, re-engagement will always be in a synchronised angular position. Alternatively multiple positions such as every 180° or 60° are possible. The recesses can be machined to give different torques according to the direction of rotation. Models are available for in-line transmission with shaft couplings, also offset with narrow or wide gears and pulleys. These popular designs are to some extent a European standard product and have ratings from 2 to 12000 Nm. 

Reversing and reciprocating drives in the packaging industry have triggered further development of ball type torque limiters with backlash free variants. These have also found use with index tables, printing machines and machine tools. The internal structure of the torque limiter is modified to remove splines and generally a locking bush connection to the shaft is used. A higher degree of sensitivity is possible too by using disc springs that have a negative spring rate. Thus as the drive plates begin to separate on overload, the spring force decreases and the release time becomes very fast indeed. Such torque limiters are more complex and costly but offer the highest levels of protection. They are typically available for torques up to 750Nm and can be supplied with stainless steel construction for severe environments. 

Generally mechanical torque limiters are set to release at the highest torque required by the drive plus a service factor. In many machines the starting torque is often the highest torque. Setting the torque limiter above this level is necessary to prevent tripping on start-up, but may result in a lack of sensitivity during the normal working part of a machine cycle. The ability to easily adjust the release torque during a cycle, or perhaps for a different product running on a machine, is an advantage contributing to a higher degree of protection. Electronic monitoring can easily handle different levels of torque, but as mentioned above does not give the disconnection. The torque limiter solution is to use pneumatic actuation. 

Pneumatic torque limiters are effectively clutches with the ability to disconnect a drive. A pneumatic piston replaces the springs of mechanical limiters, and the release torque is directly proportional to the air pressure applied. Factory air pressures up to 6 bar are used and this gives the ability to vary the release torque through a work cycle, or to quickly set a new release torque for a new machine set-up. Variable torques can be set for coilers and uncoilers proportional to the reel diameter. These roller based designs synchronise input to output and give release torques in a range of 7 to 30000Nm. 

Over the years mechanical torque limiters have been developed to suit a number of market niches. Linear force limiters trip when a set force is exceeded either in tension or compression and are frequently used as gearbox torque arms. A pneumatic friction design suits simple tensioning applications as the pneumatic cylinder automatically compensates for wear on the friction linings. Roller types can be supplied to give an overload signal without disconnecting the drive, a useful feature where the load might otherwise fall. Every application has individual requirements for protection and sensitivity. Where operator safety is a factor, higher standards are needed and advice from experienced suppliers is recommended. The new safety standard EN ISO 13849-1 requires machine designers to re-assess hazards although, as a standard related to control systems, it does not specifically deal with torque limiting. Nevertheless the review process may identify torque limiters as a solution to achieving higher safety levels. 

Mechanical torque limiters are today available in widely different forms to suit practically all types of machinery. The level of protection achieved depends on the sensitivity and repeatability of the torque limiter. Compared to electronic torque monitoring, mechanical torque limiters offer the important advantage of disconnection where the inertial energy on the drive side is isolated from the output, reducing damage and improving safety.
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