Tom Bishop, PE
EASA Senior Technical Support Specialist

This article addresses methods for removal and installation of couplings on straight or tapered shafts. The most common good practice methods will be described.

Coupling removal
When a coupling is held in place by a setscrew, completely remove the setscrew and inspect the hole. It is not uncommon to find doubled setscrews used for security. Failure to loosen a setscrew may result in a badly galled shaft and coupling bore.

In most cases, the judicious use of heat simplifies the job of removing a coupling or pulley. When using a hydraulic puller to remove a coupling, avoid directly heating the chain that keeps the puller jaws together. Use a crane or hoist when needed to avoid dropping a heavy coupling.

Induction heating is an option often used for removing large couplings or those with a heavy interference fit, such as coupling fits without keys. Specialized equipment with a variable-frequency range of typically 3 kHz to 10 kHz heats the part via insulated wires wrapped around the outside of the part. The part expands from the outside surface in and can be slid off the shaft after being heated to temperature. When a 0.002” (0.05 mm) feeler gauge of non-magnetic stainless steel can be inserted between the bottom of the part and the shaft, the part is ready for removal.

One-piece couplings can be damaged by carelessly pulling on the flange. Instead, use a puller with long arms or jaws to pull directly on the coupling hub (Figure 1). For a two-piece coupling of the “gear tooth” design, avoid pulling directly on the shroud. Instead, try to pull directly on the hub to avoid damaging the gear teeth of the hub and the shroud. If heat is required, first check for an O-ring or gasket inside the shroud (Figure 2). Remove it to avoid damage. With a two-piece coupling, slip the shroud back and inspect the hub for setscrews.

Tapered-bore couplings, fitted to tapered coupling fits, fall into two categories: mill-duty and traction-motor applications.

For mill-duty applications, the coupling is usually fitted with a key and retained by a nut. This type is best removed by loosening (but not fully removing) the nut, applying moderate pressure with the pullers and heating the coupling so that it slides off the shaft.

When removing a tapered coupling, never stand directly behind the pullers. The interference fit requires considerable pressure to remove, and once the fit releases the puller, pressure can throw the coupling and pullers a great distance. This is especially true of keyless tapered fits, which may require 10,000 psi (69,000 kPa) to remove.

The second type of tapered-bore couplings is more common in traction motor applications where the high torque demands special consider

may result in broken shafts. To avoid this, some cyclical high-torque loads utilize a tapered shaft with no keyway.

Most of these designs incorporate a hydraulic relief for removing the coupling. The interference fit is so great that simply heating the coupling and applying pressure is rarely adequate to remove the coupling. The hydraulic relief utilizes a special “needle” fitting to attach a hydraulic pump and pressurize the fit to expand the coupling. A tapped hole that is visible in the shaft end is common to a machined groove around the coupling fit beneath the coupling. In most cases, the special needle is available from the motor or equipment manufacturer.

Pump pressure is generally 10,000 psi (69,000 kPa), which requires small diameter stainless steel tubing to connect to the needle. Because of the high pressure involved, small diameters are essential to avoid bursting the pipe or fittings. Do not use seamed pipe for plumbing the hydraulics because the burst pressure is not adequate.

Coupling installation
Thoroughly clean and prepare the shaft ends. Remove any raised metal, nicks, burrs, dents, gouges, etc., prior to coupling installation. Further, all parts should be clean and free of any foreign materials before attempting installation or assembly; use a clean cloth dampened with a nonflammable solvent.

Measure the shaft diameter and the hub bore diameter to determine the interference or clearance of the fit. Check the key fit in shaft and coupling keyways. The key should fit snugly against the sides of the keyway. Insert the key flush with the end of the shaft.

Mount sleeves, seals (lightly coated with Ky Jelly or Vasoline), end plates and shrouds on shafts before mounting hubs. Locate seals on shafts so that they do not contact the hot hubs. Heat hubs to 350°F (180°C) using induction (e.g., bearing heater) or oven heating. For induction heating, carefully measure the bore with a bore gauge or inside micrometer until the clearance is 0.004”-0.005” (0.10-0.12 mm). With oven heating, set the oven to 350°F (180°C) and heat the hub at least one hour for each inch (25 mm) of wall thickness. Note: Temperatures above 550°F (290°C) may soften the hub, reducing the strength of the steel and may affect the performance characteristics of the hub.

The temperature required for expansion of the hub can also be calculated using the formulas below.

Imperial units
E = Bore expansion (in.)
D = Bore diameter (in.)
R = Temperature rise above ambient (°F)
A = Ambient temperature (°F)
T = Heated temperature (°F)
R = E / (D x 0.0000064)
T = R + A

Example:
E = 0.005” D = 3.0000” A = 70°F
R = 0.005 / (3.0000 x 0.0000064) = 260
T = 260 + 70 = 330°F

Metric units
e = Bore expansion (mm)
d = Bore diameter (mm)
r = Temperature rise above ambient (ºC)
a = Ambient temperature (°C)
t = Heated temperature (°C)
r = e / (d x 0.0000115)
t = r + a

Example:
e = 0.12 mm b = 75.0 mm a = 21ºC
r = 0.12 / (75.0 x 0.0000115) = 139
t = 139 + 21 = 160°C

Place parts to be heated on a clean steel plate. With either method, heat must be applied evenly to avoid distortion. Extreme caution must be exercised when handling heated hubs to avoid injury to personnel. Mount the hub on the shaft as quickly as possible to avoid heat loss. Make certain that the position of hub teeth (if applicable) relative to shaft end is correct before assembling. Carefully line up hub bore and keyway with shaft and key and slide hub onto shaft until hub face is flush with end of shaft.

If it is necessary to drive the hub into position, lightly tap with a dead blow hammer. Do not strike hub teeth, and avoid excessive pounding, which can cause damage to bearings. On inclined or vertical shafts, hold hubs in place until the assembled position can be maintained. Allow the hubs to cool, then insert setscrews (if applicable) and tighten.

It is important when mounting interference hubs to make sure that there is a slight clearance over the top of the key in the hub keyway; otherwise, when the hub cools, it will contact the key, resulting in high stresses in the hub that could cause it to fail. Also, there should be a snug fit between the sides of the key and the shaft and hub keyways.

The judicious application of anti-seize compounds to the shaft before installing a coupling simplifies future removal of the coupling. The exception to this is a shaft with a tapered fit. Never apply any product to a tapered coupling fit, or slippage will result.

For a tapered coupling, the fitting process is critical. This is best done with the shaft vertical and the tapered end up. Apply a light coating of Prussian Blue to the coupling fit and then snap the coupling into place. Remove the coupling and lightly hand-sand the contact points. Clean the coupling and repeat the process until a minimum of 90% contact is achieved.

DO NOT TAKE SHORTCUTS! One so-called “shortcut” was to use lapping compound to fit the tapered shaft and coupling. The abrasive lapping compound will wear down the portion of the shaft within the coupling while leaving the rest of the shaft at its original diameter. This creates a step on the shaft (Figure 3). When the coupling is heated and installed, the only tight fit is where the coupling is advanced onto the step. The remaining interference fit is drastically less than required, and the coupling will spin when subjected to a torsional load.

Follow the manufacturer’s guidelines when installing the coupling. Coupling temperature must be measured against shaft temperature when determining the proper advance. To confirm that the coupling is advanced the correct distance:

• Place the coupling (ambient temperature) on the shaft and measure the gap behind the coupling.
• Subtract the desired advance and place a spacer of the appropriate size to serve as a stop.
• If the shaft end has a tapped hole, prepare a retainer to prevent the coupling from creeping off the taper while cooling.
• Leave the retainer in place until the coupling and shaft temperatures are equal.

AVAILABLE IN SPANISH

Categories: Technical topics, Miscellaneous, Couplings

Tags: Couplings

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