As a machine designer, one of your top priorities should be to protect against operator injury and machinery damage due to overloads. Limiting the torque that can be transmitted through a system is one way to do it. A variety of torque limiters can do the job: from basic shear pins to friction and ball detent units, some with pneumatic or electrical controls.
A steel shear pin fitted between two mating components of a machine acts as a mechanical fuse to prevent damage caused by excessive load. When a jam occurs in a rotating machine, one of the components stops, causing an overload. If the overload exceeds the pin’s shear strength, the pin breaks, thereby separating the two components and protecting the machine from damage.
Resetting the device requires replacing the pin — a simple procedure unless the sheared pin has damaged the mating components so they must be repaired or replaced. Though this method is inexpensive, it may be difficult to accurately match the pin shear strength to the required load.
Shear pins are commonly used in mining equipment, where machines are often too large for other types of torque limiters.
Friction torque limiter
For many applications, a friction type torque limiter is more practical. This device operates similar to the brakes on your car. A drive component, such as a belt pulley or gear, is sandwiched between two friction linings of the torque limiter. Belleville disc springs or coil springs apply pressure through a control element to the friction linings so they grip the drive component.
When an overload occurs, the resultant high torque causes the drive component to slip within the friction linings, thus preventing damage to the machine. When the overload is removed, the spring pressure causes the device to once again transmit torque.
Spring pressure can be adjusted to accurately obtain the required slipping torque for the machine, so its response is more predictable than that of a shear pin. To maintain the torque capacity, do not lubricate the friction surfaces. Eventually, the friction linings wear out, but they are easy to replace.
Some friction torque limiters have bronze bushings upon which the bore of the component rides, others have a needle bearing in place of the bushing to accommodate large radial loads on the driven component.
As a simple, low-cost method of overload protection, the friction torque limiter is typically used in applications such as sprocket drives on packaging equipment, conveyors, and agricultural machinery.
Ball detent torque limiter
The ball detent torque limiter operates with a series of balls (or rollers) in one side of the device (control element) that rest in mating sockets or detents in a pressure flange on the opposite side. The control element attaches to the driven shaft, and the driven component attaches to the pressure flange (sometimes called thrust flange). The entire device rotates with the driven machine until an overload occurs, at which point the two mating halves of the device move axially apart against the spring force. The balls slide out of the detents, letting the two halves rotate relative to each other, and they attempt to re-engage in the next set of detents. This action continues until the overload is removed.
Ball detent torque limiters typically require lubrication of the balls or rollers. This type of limiter accurately disengages at a preset torque value, and its response is more predictable than with other torque limiters. Accuracy, plus higher torque capability, makes the ball detent device suitable for applications where precision is a must: packaging, wood working, textile and food processing equipment.
There are several versions of detent devices:
• Basic ball detent — a series of balls in mating detents. When an overload occurs, each ball rotates out of its detent and moves toward the next detent. If the overload is removed, each ball seats in the next detent, thereby resetting the device so it is ready to transmit torque. If the overload is still present, the ball moves toward each consecutive detent until the overload is removed. When the overload is removed, the unit resets itself, with each ball seating in the next available detent. This action is termed “random automatic resetting.”
• Roller detent — a series of rollers in mating detents. Upon overload, the rollers slide out of their corresponding detents and continue rolling in the direction of rotation until the overload is removed. Typically, the unit must make a complete revolution (360 deg), then it resets itself if the overload is no longer present. This is called “single position automatic resetting.” If the overload is still present, the unit continues to rotate until the operator stops the machine and removes the overload. Resetting is accomplished by rotating the machine either clockwise or counterclockwise until the rollers re-engage in their detents. Optional reset positions (such as 45, 90, and 180 deg) are available.
• Torque sensing — this device does not slip in a rotary direction. Its two halves slide axially away from each other when an overload occurs, but the balls do not disengage from their detents. Axial movement of the two halves trips a limit switch or proximity switch that turns off the machine or alerts an operator.
Because the balls don’t disengage, the unit continues to transmit torque, acting as a fail-safe device (to prevent a load from falling, for example). This type of unit is useful in applications where it is not desirable or safe to have the torque limiter disengage, such as cranes or inclined conveyors.
• Free wheeling — in this device, the two mating halves move axially away from each other and disengage when an overload occurs, but the device must be manually reset. Re-engagement occurs at a random location unless a predetermined re-engagement location has been specified.
Cam follower device
The spring-loaded cam follower or pawl-detent device is an older, yet reliable overload device. It operates similar to a ball detent device, but uses cam followers rather than balls. When an overload exceeds the preset torque limit, each follower slides out of its detent, disconnecting the two halves of the device. When the overload is removed, the operator manually re-engages the device by jogging the machine forward or reverse until the followers seat in the detents again.
Like ball detent devices, this type is available in several configurations including single or multiple position, chain coupling (shaft to shaft connection), and with overload detector mechanisms for using limit switches.
Cam follower units are commonly used in sewage treatment plants, where an enclosed unit protects the pumping and aerating mechanisms from jams.
Some torque limiters minimize or eliminate internal backlash for applications requiring precise repeatability. These devices use special methods for spring-preloading the control element and thrust flange, and tighter tolerances on mating components to eliminate internal backlash. Using a keyless locking device to attach the unit to the shaft also reduces backlash.
As electronic and pneumatic controls have become more reliable and less expensive, they have gained acceptance for use with torque limiters.
Such systems use electric current or air to apply pressure on the thrust flanges and control elements of a torque limiter. By controlling the electrical current or air volume, you can adjust the torque setting to accommodate the machine cycle requirements.
Adjustable torque is desirable, for example, where the startup torque of a system is much higher than the running torque. It’s also useful where one machine processes different materials that require different amounts of torque. The operator can adjust the settings so the torque limiter slips only when there is truly an overload. Some systems offer a random reset device or a single position device similar to the manual ball detent devices described previously.
PNEUMATIC. A simple pneumatic control system uses air pressure on a piston (rather than a spring) to apply force to a ball detent device. In one version, air pressure causes the torque limiter to engage so it can transmit torque. In the event of an overload, the control system releases (dumps) the air and disengages the device so it no longer transmits torque. Accidental air loss also disengages the unit, acting as a machine safety feature.
Another version transmits torque in the absence of air. When an overload occurs, relative movement of the two halves of the torque limiter actuates a limit switch, causing air to disengage the unit.
A typical system has single air pressure control, and allows the operator to manually adjust the torque setting. For drives that require a high start-up torque and a lower running torque, such as a conveyor belt that is fully loaded at startup, a dual air pressure system is used. Dual systems can be programmed to electronically actuate control valves in a prescribed sequence that suits the application. A pneumatic system also offers safe operation in explosive or magnetically sensitive environments.
ELECTRIC. Similar in function to pneumatic systems, electromagnetic torque systems use electric current to energize a magnetic coil that engages or disengages the torque limiter. These systems are either spring set and electrically released, or electrically engaged and spring released. Each is used depending on safety requirements for the machinery or personnel.
LIMIT SWITCHES. Advances in limit switch technology also simplify the use of many torque limiters. From traditional mechanical switches with large strokes to advanced mechanical switches with very small strokes and electronic proximity switches (optical or magnetic), designers have many options. In dusty environments, for example, a mechanical switch may function better than an optical proximity switch. In explosive environments, on the other hand, a sealed proximity switch is better than a mechanical switch that requires special sealing and spark isolation.
Where do I put it?
Selecting the best location for a torque limiter can be tricky. Ideally, you should place the torque limiter next to the component or process that needs to be protected. If a machine is subject to stalling or jamming, placing a torque limiter between a gearbox (speed reducer) and the driven machine protects the gearbox from overloads as well as the motor. However, this location is at the low-speed side of the gearbox, where torques are higher. Therefore, a larger, more expensive torque limiter is needed than if the limiter was installed on the motor output shaft. The tradeoff is that a smaller torque limiter can be placed between the motor and gearbox to save cost, but the reducer will now experience any overloads from the driven machine. Nevertheless, safe operation of the system should dictate the location of the torque limiter.
Carl W. Fenstermacher is the president of Ringfeder Corp., Westwood, N.J.