Torsional Holding Power

The primary function of a rigid shaft coupling is to join two shafts such that they will function as one, retaining both the linear and angular relationship between the two shafts.

The amount of torsional load (inch-pounds or newton-meters) required to initiate slippage of a shaft in a properly installed coupling is defined as the torsional holding power (or torque rating).

Torsional Holding Power is a function of the following:

1. 1) Tension induced into the screws by proper seating torque.
2. 2) The coefficient of friction between shaft and coupling. (Smoothness of the friction surfaces, plating, lubrication, etc.).
3. 3) Clearance between shaft and coupling prior to screw tightening.
4. 4) The smaller diameter of the two shafts.

Because there are so many variables involved in determining the actual torsional holding power between a specific coupling and shaft, it is not possible for Stafford to certify a single value for all situations. Instead, we can provide results that have been obtained from testing under a controlled set of conditions that most accurately represent a standard operating environment. These results should be used as a guide only for projected shaft coupling performance, and should not, under any circumstances, be used as guaranteed values. Users of these guides do so voluntarily, as a reference, aiding them in testing and proving the coupling’s suitability in their particular application.

Axial Hold is the amount of axial force required to initiate axial slippage between the coupled shafts. It also is highly dependent on screw tension and coefficient of friction between shaft and coupling. This is provided as a reference only.

Examples are for Axial or Torsional loads NOT for combined loads.
 

NOMINAL TORSIONAL HOLDING POWER

Alloy Screws in Steel Couplings

NOMINAL TORSIONAL HOLDING POWER

Stainless Screws in Stainless Couplings

NOMINAL TORSIONAL HOLDING POWER

Alloy Metric Screws in Steel Couplings

NOMINAL TORSIONAL HOLDING POWER

Stainless Metric Screws in Stainless Couplings

USEFUL APPLICATION INFORMATION

• Keyways: Stafford offers many couplings with keyways. Keyed couplings provide additional torque but we do not publish torque data.
• Aluminum Couplings: 2024 aluminum is a strong alloy, but it is softer than the 1215 steel shown in the above examples. For both Alloy and Stainless screws in aluminum couplings, the screw torque should be 50% of that shown for alloy screws in 1215 couplings and the resulting torsional hold will also be 50% of that shown for the 1215 couplings.
• Stepped Couplings: All the above data are for straight thru couplings. The torsional hold of a given size coupling will reduce in a linear fashion to that of the smaller bore. Thus if one has a 1 X ¾ coupling, the torsional hold will be ¾ of that shown for the base 1 X 1 coupling. For 1-1/2 X 1, the resultant hold would be 2/3 of the base 1-1/2 X 1-1/2.
• Shaft and Coupling Prep: At a minimum, the shafts and coupling bores should be wiped clean of any foreign matter or excess oils. The clamping screws have an oil coating, which should remain.
• Installation: Insert shafts and tighten screws until the fit is snug but the shaft can still be rotated slightly inside the coupling. Rotate just a bit to ensure proper seating. Tighten all screws in small steps and sequentially, until all screws are at the recommended seating torque.
• Additional Torsional Hold
  • Keyways: Stafford offers many couplings with keyways – these will carry whatever torsional load the key will support and provide positive radial location.
  • Larger Screws: Special couplings (or our Heavy Duty Clamp-Type Couplings), with larger screws will offer added torsional hold.
  • More Screws: Special couplings with more screws will offer added torsional hold.
• Additional Axial Hold:
  • Radial Keyways: Radial keyways on shafts and couplings will supply greatly enhanced axial hold as well as a positive and repeatable axial location.
  • More and Larger Screws
• All examples are for static loads
• Galling: Galling can be an issue with excessive tightening torque in stainless fasteners when installed in stainless couplings. If this occurs, the screw and coupling can weld together and both become unusable. The above results were obtained with screw torques that are reduced to minimize the possibility of galling. If appropriate to the application, the user may opt to apply a high pressure lubricant to the screws to allow for higher screw torque and thus higher coupling torsional hold.