Keeping machines breakdown-free

Sept. 1, 2000
Heavy-duty machines that operate under severe conditions use fail-safe hydraulic couplings to keep sudden unexpected overloads from inflicting major damage.

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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On the firing line

Hydraulic couplings have been applied to drives in steel mills, power generation systems, paper mills, pumping systems, and material handling systems where equipment is very expensive and the cost of downtime is high. Two of the most common applications are steel mill drives, where overloads usually result from mechanical malfunctions, and gas turbine generators, where the excessive loads have electrical causes.

In steel mills, overloads occur on the main drives of various types of equipment used in hot and cold strip, tube, rod, bar, and wire mills. Specific machines include large mill stands, reducers, straighteners, coilers, cutters, levelers, and material handling equipment.

The high loads are usually caused by sudden-acting cobbles, overdrafts, and cold slabs. A cobble means that the steel folds over on itself and wedges between the work rolls. With an overdraft, the machine reduces the raw material in size more than intended, and this requires too much torque. A cold slab is a strip of steel that isn't hot enough to allow the intended bending and shaping.

When excessive loads occur in a machine without a torque limiter, rolls or drive spindles break, shutting down the equipment and creating a hazard for operating personnel. The risk to personnel comes from the speed at which components break and fly apart, as well as from their large size and heavy weight. Repairing the damage from such an event may cost several hundred thousand dollars.

A hydraulic coupling works well in such situations because it reacts in less than a second after an overload starts, preventing most, if not all, of the equipment damage.

Other common applications involve power generation systems containing generators and aircraft-type gas turbines modified for industrial purposes. Downtime in these systems is very costly, and unscheduled shutdowns can disrupt critical operations.

Malfunctions usually originate as electrical faults in the generator, causing large torsional loads to travel backwards in the system, thereby damaging the turbine, gearbox, and other PT components.

Protecting a gas turbine system requires a torque limiter that trips within a small safety range, which is generally 2 to 2.5 times the full-load torque. Hydraulic couplings suit this type of application because they can be set at torque levels that are low enough to stay within these safety ranges. Furthermore, they consistently release at a torque close to the preset level, thereby staying within the safety range.

Controlling friction is the key

Hydraulic torque limiting couplings incorporate two features to ensure a consistent frictional grip between shaft and hub. First, they have hardened friction surfaces to prevent wear or seizing when the coupling releases. This provides a reliable connection after repeated engagement and disengagement cycles. Typical hardening methods include ion nitriding and bronze coatings. Second, the surfaces are polished to a smooth finish to obtain more contact area.

These features combine to ensure that the coupling securely grips the shaft and hub until the preset torque limit is reached, then disengages at that point.

Gas turbines need more

Besides requiring protection from torque overloads, some gas turbines need overspeed protection. Overspeeding occurs when a gas turbine is mechanically disengaged from the workload. Then the higher inertia of the generator causes the turbine to momentarily increase speed. Residual energy in the system also affects the speed.

One way to control overspeed is to incorporate a hydrodynamic coupling along with a hydraulic torque limiting coupling. The hydrodynamic device uses a fluid power mechanism to damp fluctuations in speed and torque between the generator and turbine. When the drive mechanically disengages, the turbine speed decreases with that of the generator and overspeeding doesn't happen. The damping also reduces system vibration.

Todd Clark is an applications engineer, Voith Transmissions Canada Inc., Concord, Ontario, Canada.

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