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Engineer’s Angle: Choose correct coupling for the job

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Engineer’s Angle: by Rich Merhige

Couplings are used to connect two rotating pieces of equipment together at their ends for transmitting power. There are two types of couplings: rigid or flexible.

On yachts, rigid couplings are used to join shaft ends together or connect the shaft to the reduction gear. Flexible couplings are used to connect the engine to the gearbox and, in some cases where great vibration reduction is desired, to connect the gearbox to the propeller shaft. Flexible couplings typically allow some degree of misalignment or end movement or both, and they can also help in vibration isolation.

Muff couplings and flange couplings are rigid couplings used on yachts. Muff couplings are the simplest type and are used to join the ends of propeller shaft sections together. In essence, it’s a pipe whose bore is in line with the tolerance of the shaft’s size. Muff couplings can be a sleeve type or a split type.

Sleeve type muff couplings consist of an inner and outer sleeve tapered to match each other. The outer sleeve is driven onto the inner sleeve to clamp the two shaft ends together. This can be done hydraulically or mechanically.

Advantages of a sleeve coupling are that it does not require a keyway, which weakens the shaft.

The area where the coupling and shaft end make contact must be clean and free from burrs. Even the smallest particle trapped between the shaft and bore can make it unsuitable for use. Any nicks, scratches and edges on mating surfaces should be polished smooth. When blue-fitting or lapping hydraulic couplings to propellers, there should be a minimum of 85 percent contact.

During inspection, o-rings should be checked for cuts and abrasions. The coupling needs to be kept clean and sealed from seawater, either with a shrink fit or fiberglass cover. The shafts and couplings joined together are a precision fit, so any corrosion or fretting can result in the coupling becoming seized on the shaft, requiring it to be cut off, destroying the coupling in the process.

Split-type muff couplings are made of two halves joined together. Keyways are machined into the coupling’s bore as a means to help transmit torque and prevent the shaft from rotating in the bore.

Advantage of a split coupling is that the shaft position does not need to be changed to assemble or disassemble the coupling. When assembling this coupling, it is important to follow the correct tightening procedure to avoid an eccentricity of the coupling or shafts or having the coupling come loose.

Flange couplings are typically used in marine propulsion drivelines to connect the propeller shaft to the reduction gear. One half of the flange is mounted to the propeller shaft and the other half is mounted to the gearbox output shaft. The faces are turned up at right angles to the axis of the shaft and the bore is keyed to the shaft. One half of the flange has a projected portion, called a pilot, and the other has a corresponding recess. The pilot helps to bring the shafts into line and to maintain alignment. The two flanges are coupled together by means of bolts and nuts. It is good practice when performing running gear service to mount the coupling on the shaft and verify the pilot diameter and check the runout for eccentricity. Any runout should be corrected to avoid shaft vibration.

Flexible couplings are introduced in a driveline to reduce vibration. To do this, the flexible coupling is installed between the engine and gearbox. It allows for varying degrees of misalignment and even parallel misalignment. They also control the torque vibrations within the system.

This type of coupling is specifically used to protect the driving and driven shafts against misalignment, sudden shock loads, shaft expansion or vibration. They are a bit more complex than the aforementioned couplings, and need to be precision aligned.

Flexible couplings allow for misalignment and introduce mechanical flexibility into the system. Couplings also work to dampen noise and to protect other mechanical components from overload.

Flexible couplings can be aligned in place or by using a dummy. To align them in place, the hubshaft needs to be disconnected so that the propeller shaft can be moved out of the male/female connection. The axial and radial alignment needs to be checked against the gearbox flange.

Generally speaking, you can tell if a coupling is properly installed if it’s easy to connect or disconnect.

The manufacturer’s instructions and recommendations should always be followed. Couplings should be cleaned regularly and visual inspections conducted to see signs of wear and fatigue. If required, any lubricant in the coupling should be checked and changed regularly if it’s a lubricated coupling.

Like all machinery, couplings must be maintained to maximize their lifespan and to prevent unnecessary failure. These couplings commonly contain rubber elements or springs that need to be regularly maintained or replaced to ensure they properly transmit torque.

Failures can happen with improper installation, incorrect coupling selection, or operating the machinery beyond the coupling’s design capabilities.

Signs of damage and failure include: excessive vibration, abnormal noise, a visible wobble, leaking or failed shaft seals. A vibration analysis is an effective way to avoid or provide early detection of the coupling condition and avoid failure.

Rich Merhige is owner of Advanced Mechanical Enterprises and Advanced Maintenance Engineering in Ft. Lauderdale (www.AMEsolutions.com). Comments are welcome below.

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