Oil pressure forces a hydraulic coupling into frictional engagement with the motor shaft and a hub that bolts to the driven machine. When the torque exceeds a preset limit, the shaft starts to turn along with a ring that cuts the shear tube. This action releases the pressure-induced frictional grip, disengaging the motor from the machine.
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Certain types of machines require drives to operate in continuous heavy-duty service and severe environments. For example, steel rolling mills operate under the most severe conditions imaginable, applying tons of pressure on slabs of steel to squeeze them into long thin plates or sheets.

These machines must withstand unpredictable malfunctions in their operation, such as a jammed line, that cause torque transmitted by the drives to jump sharply. Such malfunctions typically cause some drive components to suddenly stop moving, which imposes extremely high torque overloads on the equipment, leading to extensive damage. Such machines need torque limiting devices that quickly react to these malfunctions to prevent the huge loads that would otherwise mangle drive components and put the equipment out of commission for hours or days.

Potential solutions

You can protect against slow-acting overloads with electrical equipment such as motor controls. However, rapid overloads occurring in less than a second require fast-acting mechanical torque-limiting devices to protect machines and safeguard operators.

Torque limiters relieve excessive loads by interrupting the connection between driving and driven machine at a preset torque level. Some do this by frictional slipping or by displacing one or more parts (balls in a ball detent device) against a spring force. Others do it by breaking a sacrificial element, usually a shear pin, mounted between two mating drive components. Depending on the type of limiter, you can reset it to transmit torque again by replacing or repositioning parts. Certain types automatically reset.

Some conventional torque limiters are not available in the large sizes required for equipment such as steel rolling mills and gas turbines. Therefore, these machines generally use shear pin couplings.

Shear pins often bend rather than break during a release, and they can be reused. When the pins are eventually replaced, it may take several hours to reset a coupling because the bent pins are difficult to remove. Repeated cyclic loading weakens the pins so they release at lower and lower torques, below the preset design limit. For this reason, many shear pin releases occur unnecessarily.

A better way

Another option for heavy-duty machines is to use hydraulic torquelimiting couplings, which accommodate shaft sizes up to 40-in. OD. These devices release or disengage like a traditional shear pin coupling by destroying a consumable component, in this case a shear tube. But they differ in several ways. First, they adjust over a large torque range, up to 60 million lb-in. for some units.

Second, they consistently release to within ±10% of the preset torque limit. Unlike shear pins, the shear tubes don't experience cyclic loading and can't be reused. So they don't get weaker with time and there are no unnecessary releases at torques lower than the preset value. Third, the devices can be quickly reset. Depending on their size, the reset time ranges from 5 to 40 minutes.

How it works

A hydraulic torque-limiting coupling consists mainly of a hollow twin-walled flexible steel sleeve that fits between the motor shaft (or gearbox shaft) and a hub. The hub in turn bolts to a flange on the driven machine shaft. Oil pressure applied to the sleeve, using a hand pump or motorized pump, causes the sleeve walls to expand and come into frictional contact with the shaft and hub.

The resultant frictional force lets the shaft transmit torque through the sleeve to the hub. The selected oil pressure determines the maximum torque the coupling can transmit before slipping occurs between the sleeve and shaft.

If the coupling experiences a higher torque than the preset limit, its frictional grip is overcome and the shaft slips and rotates within the sleeve. Because of this relative movement, a ring fixed to the shaft cuts off the top of one or more shear tubes connected to the sleeve, causing an instantaneous drop in oil pressure. Consequently the transmitted torque drops to zero, letting the motor and driven machine rotate independently.

After an overload release, operators replace each shear tube and repressurize the coupling to again transmit torque.

To ensure that friction surfaces don't contact each other after a release, rolling element bearings are fitted between the shaft and hub. These bearings let the coupling operate up to several thousand rpm, depending on the coupling size. Plain bearings are sufficient for couplings operated at speeds below 900 rpm, again depending on size.

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