Modern machines can be simple — a single element translating a rotary-to-linear motion. They can also be complex — a collection of interacting mechanisms operating in a multi-dimensional plane. The commonality of these mechanisms is the element of motion that is predictable with a known torque or force and velocity.
Circumstances may occur where a mechanism encounters an unexpected force, such as a jam or tool breakage, that could exceed the mechanism design limits and result in possible damage to the machine, work piece, or machine operator. By executing a controlled decoupling of the unexpected force, torque limiters are key in eliminating potential mechanism failure or injuries.
Benefits of Torque Limiters
A critical requirement in the design of different types of machines is protecting against damage and downtime caused by machine overloads. Torque limiters address this challenge by behaving as 1:1 transmission, as long as torque is less than a specified value. When the torque surpasses the limit, torque limiters operate as a clutch to quickly disconnect the drive from the driven system, removing much of the inertial energy from the drive train — typically in milliseconds or fractions of milliseconds. Applications for torque limiters include packaging machinery, conveyors, assembly lines, machine tools, woodworking machines, textile machinery, industrial robots, sheet metal processing equipment, printing and converting machines, and servo and DC motor drives.
The drive torque is the most important criteria for selecting a torque limiter. In multi-component mechanisms, the placement of the torque limiter determines which components are protected. The torque limiter can be located at various points along the drive train and set to decouple the downstream components from the load. Placing a torque limiter between the output of a gearbox and the driven load protects both the gearbox and motor from overload; however, if the device was placed between the gearbox and the motor instead, a lower torque rated limiter could be used at the cost of only protecting the motor. Best practices locate the torque limiter to protect the most expensive component of the drive train.
The torque load determines the size, type, and cost of the torque limiter. Ideally, the torque limiter should be selected or adjusted to slip at high enough torque to avoid unnecessary tripping, such as during initial machine startup, which is typically the point of highest torque under normal operating conditions. For extended service life, it is best to keep the torque limiter operating where the shaft speed is relatively low.
Types of Torque Limiters
The simplest type of mechanical torque limiter is a shear pin linking two rotating bodies and designed to break at a certain torque level to separate the drive system from the load.
Friction torque limiters operate similarly to automotive brakes — a drive component sandwiched between friction linings is connected to a driven component that grips the drive component during normal operation. An overload causes the drive component to slip relative to the friction linings, thus protecting the machine from damage. Removal of the overload causes the drive to resume transmission of torque.
The ball detent style of torque limiters uses a series of balls or rollers positioned in mating sockets (detents) to connect the drive and driven elements and held in position under normal operating conditions by spring force. When a torque overload occurs, the balls/rollers overcome the spring pressure and force the drive and driven element apart. The balls/rollers slide out of the sockets (disconnecting the driving elements from the driven elements), which allows them to rotate relative to each other in tracks around the plates. More advanced ball detent torque limiters use pneumatic technology instead of springs to engage the torque limiter. An advantage of the pneumatic approach is that the trip-out torque can be remotely controlled by using air pressure adjustment to increase or reduce the set torque.
Hydraulic torque limiters apply hydraulic pressure between the drive and driven elements of the torque converter to connect the power source to the load. When the input load exceeds the desired release torque (which is accurately controlled by the operating pressure level), the hydraulic pressure is no longer sufficient, causing the driven element to slip against the driving element. Hydraulic torque limiters are primarily used in very high torque applications.
Magnetic torque limiters can provide advantages in certain niche applications. In this type of device, the drive and driven elements do not touch each other but are instead connected through electromagnets that exert force at a distance. They are also used to reduce the transmission of torsional vibrations. Magnetic torque limiters do not wear, do not require lubricant, and can be used at nearly any temperature.
An alternative approach is to electronically monitor the torque output at the motor and apply a braking torque when an overage is detected. This approach is well-suited in situations where the load increases in a linear fashion at a relatively slow rate.
The example show in the diagram on the opposite page is a Nexen TL torque limiter, a pneumatically engaged ball/detent clutch equipped with proximity overload sensors. Trip-out torque is remotely set with an air regulator using torque vs. air pressure charts. The TL is set in the run condition by jogging the shafts until the balls engage in the detents. A hard-chrome detent interface decreases drive ring wear when the balls are pressed against the face during the jog-to-position engagement step.
A proximity sensor, positioned next to a dowel pin embedded in the TL air chamber, sends a signal to the pneumatic, directional control valve to exhaust air from the air chamber when an overload occurs. Internal springs separate the interfaces at disengagement, preventing the ball from being forced out of the detent in an overload condition, extending TL life, preventing detent distortion, and preventing the TL from trying to re-engage until the machine is stopped. If the torque limiter does not totally disconnect, the detents could be damaged enough to affect the subsequent trip-torque setting.
Mechanical torque limiters help the machine designer to protect the equipment and operator against damage due to overloads. Available in a wide range of styles and sizes to suit practically any application, selecting the right torque limiter for the application can help the machine designer protect against damage at a relatively low cost.
This article was written by Broc Grell, senior applications engineer at Nexen Group, Vadnais Heights, MN. For more information, visit here .