Couplings for connecting shafts (shown in Fig. 1) can be placed in two categories: keyed and keyless. Similarly, the fits of couplings to shafts can be categorized as clearance or interference. A keyless coupling arrangement will always have an interference fit, while a keyed arrangement may have either a clearance or an interference fit. This article will focus on determining when to use a clearance or an interference fit, and how to obtain the tolerances for the resulting fit. Since we are dealing with existing shafts and couplings, the category of keyed or keyless has already been established and will not be considered.
Clearance versus interference fits
The lower the power rating (actually the lower the transmitted torque), the greater the probability of using a clearance fit between coupling and shaft. Conversely, higher power and torque usually requires an interference fit. Further, as the power and torque requirements become very high, it is more common to find that keyless fits are required. Note that the terms "lower," "higher" and "very high" are all relative, with no guideline levels associated with them.
The questions that often arise are on when to use a clearance fit versus an interference fit, and what tolerance range to use for the applicable clearance or interference. We'll proceed from applications where a clearance fit can be used, to those requiring progressively greater interference fits. A general rule of thumb: There's little risk with having more interference than necessary, while there's more risk in having less.
Clearance fit (keys and setscrews)
Keyed clearance fit couplings are most frequently used on lower-power applications with shafts under about 2.5 in (about 65 mm). A visual indicator that the fit is clearance rather than interference is that set screws are usually provided over the key when a clearance fit is used (see Fig. 2). The set-screw location "A" of Fig. 2 is more common, but some manufacturers use location "B." Be sure to remove set screws before trying to remove a coupling. Table I reflects clearance fits used for NEMA-frame-size motor shafts.
The design principle of the clearance fit is that the torque is transmitted through the key, minimizing any sliding of the coupling on the shaft. Whether a clearance fit is acceptable depends on the torque to be transmitted, the coefficient of friction used, the dimensions of the hub and operating speed. If the torque forces or misalignment are excessive, the coupling hub may rock and become loose, leading to fretting. Evidence of this condition will be a fine rust-colored powder at the ends of the coupling fits, often with visible wear to one side of the key and/or keyway.
Interference fit (keys)
Keyed interference fit couplings are commonly used for applications up to a few thousand horsepower/kW, and speeds up to or slightly above 10,000 rpm. The interference fit standard for most couplings made of medium-carbon steel are 0.00050-0.00075 in/in (mm/mm) interference up to 1800 rpm and 0.00075-0.00100 in/in (mm/mm) over 1800 rpm. Table II reflects interference fits used for NEMA-frame-size motor shafts. The purpose of the interference fit with a keyed shaft is to axially locate the coupling hub and resist forces associated with unbalance and misalignment. A frequently used reference for keyed-coupling interference fits is ANSI/AGMA 9002-B04.
Interference fit (keyless)
For high-horsepower/kW and high-speed applications, ANSI/AGMA 9003-B08 or equivalent fits (straight and tapered) are commonly used. The interference for keyless fits needs to be adequate to withstand expected normal and transient loads. Common keyless interference fits range from 0.0015 in/in (mm/mm) to 0.0020 in/in (mm/mm). The Brinell hardness of the hub material is a significant factor in keyless coupling fit. Typical fits for various Brinell hardness (BH) steels are: 0.00175 in/in (mm/mm) for 250 BH, 0.0025 in/in (mm/mm) for 300 BH and 0.0030 in/in (mm/mm) for 330 BH.
Coupling bore and key
The key is a critical element in successful transmission of torque. The bore and keyway in a coupling hub should be checked before installation. Bores should have surface finishes of 63 to 125 microinches (1.6 to 3.2 micrometers) and must not be eccentric or skewed. If a bore is machined eccentric to the hub axis, the coupling eccentricity may cause vibration. A coupling bore machined askew to the centerline axis will increase the misalignment for which the coupling, shaft and bearings must compensate. The keyway should be cut square and centered to the shaft.
The fit of the key is critical in assuring sufficient capacity of the shaft-to-coupling hub interface. Be sure to check that the key fits tightly in the shaft keyway; that the key has a sliding fit (but not be too loose) in the coupling hub keyway; and that the key has a clearance of 0.003 to 0.020 in (0.08 to 0.51 mm) with the hub keyway at the top of the key.
The key should have chamfered corners so that it fits in the keyway without riding on the keyway radii. A loosely fitted key can roll or shear when heavily loaded and provide a path from which coupling lubricant can leak. Conversely, too tight a fit will make assembly difficult and increase residual stresses, possibly resulting in premature failure of the coupling hub and/or shaft. A key that is too high in the keyway also could cause the coupling hub to fracture.
Steel coupling hubs require an increase of 160 F degrees (90 C) for every mil (0.001 in or 0.025 mm) of interference divided by the hub inside diameter. For example, a steel hub with a 2.125 in-bore with an interference of 0.0015 in will require an increase of 1.5/2.125 x 160 = 113 F degrees (63 C). Thus, if the shaft temperature is 70 F (21 C), the hub temperature must be at least 183 F (84 C). This does not account for potential cooling due to handling time, so as a general rule add about 60 F degrees (33 C) to the calculated expansion temperature to account for these factors. In this example the target temperature would be 243 F (117 C). If the calculated target temperature exceeds 350 F (177 C), check with the coupling manufacturer to be certain the required temperature will not affect the coupling hub integrity.
The hub should be heated on an induction-type bearing heater or in an oven; a torch should not be used. Use of a torch or open flame could cause distortion or a reduction in hardness and strength of the hub material. Before installing the coupling hub, make certain that the inboard and outboard ends have been identified. Removal and reinstallation of an incorrectly installed coupling hub will be difficult or impossible at the installation location. Also, make certain that the grease seals are in place and not damaged before installing the coupling covers.
Don't simply assume that a new coupling is dynamically balanced. Not all couplings are pre-balanced by the manufacturer—and some are not designed to be balanced. Check with the coupling manufacturer or supplier to determine if a new coupling has been factory balanced. A good practice is to measure vibration levels after replacing a coupling (whether or not it has been balanced) to confirm that the levels are within acceptable limits. The length of the key will affect the balance. To determine the correct key length, add the length of the shaft keyway to the length of the coupling-hub keyway and divide by two.
The grease used for couplings is not the same as that used in electric motors. Be sure to use a coupling grease equivalent to that specified by the coupling manufacturer. Lubricant should be replenished when the motor alignment is periodically checked, and the customer should typically replenish the coupling grease on an annual basis. MT
Tom Bishop is a technical support specialist for EASA, headquartered in St. Louis, MO. Telephone: (314) 993-2220.