Page 283 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Gear and Coupling Best Practices Be st Practice 4.9

Fig 4.8.5 Multiple, convoluted diaphragm-spacer coupling (Courtesy Fig 4.8.6 Reduced moment convoluted (wavy) diaphragm spacer
of Zurn Industries) coupling (Courtesy of Lucas)

Best
Best
Best Practice 4.9Practice 4.9Practice 4.9Practice 4.9
Best
1
Limit shaft end tapers for hydraulic fit couplings to / 2 " per Lessons Learned
foot or less to allow sufficient tolerance in coupling axial 3 / 4 " per foot tapers have caused catastrophic shaft, cou-
movement (axial push or drive) to attain shrink fit. pling and coupling guard failures on small diameter shafts
Hydraulic fit couplings use only the shrink fit between the hub and because they did not allow sufficient tolerance to prevent
shaft to carry the full transmitted torque load (power/speed). coupling slippage.
If the coupling is not assembled with the proper shrink fit it will slip,
possibly causing catastrophic damage and exposing the site to Benchmarks
a safety incident. This best practice has been used since 1975 when I experienced
The shaft end taper and the axial coupling hub movement (push) 3
dimension determine the coupling shrink fit. a catastrophic coupling failure due to the use of a / 4 ” per foot hydraulic
coupling shrink fit. The use of this best practice has resulted in no
Coupling shaft end tapers for hydraulic fit vary from vendor to
vendor and even from machine to machine for the same vendor. The coupling shrink fit slippage issues since that time in all associated
projects.
greater the shaft taper, the less the push required, and the lower the
torque transmission capability of the coupling system.
3
High shaft end tapers ( / 4 " per foot and above) do not allow suffi-
cient tolerance in the event that the coupling drive (axial push) specified
values are not met.

B.P. 4.9. Supporting Material does not require coupling removal to remove shaft components
(seals, bearings, etc). Keyed fits are usually used on motors,
gearboxes and most pumps and small steam turbines. Since heat
Coupling installation and removal is usually required to remove keyed couplings, they will not be
used where removal in the field is necessary. In these applica-
The most common methods of coupling attachment are: tions either spline or hydraulic fits are used.
Spline fits consist of a male (on the shaft) and female (in
Key fit
Spline fit coupling hub) finely machined mating gear teeth with line to line
Hydraulic fit fit (no backlash). When assembled on the shaft, the fit is rigid
and provides no flexibility. Spline fits are commonly used in the
Key fits are used whenever possible. They are the most gas turbine industry. They do not usually require heat for
common method of shaft fit. It is important to ensure keys and removal.
keyways are properly manufactured to avoid problems with Hydraulic fits are used where heat to remove the coup-
removal or breakage. Key fits will be used on equipment that ling hub is either not available or not permitted. Usually,

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