Is Going Keyless The Answer?

Is Going Keyless The Answer?

How do you connect a power transmission component or other item to a shaft? There are three traditional approaches: shrink fit/press fit, material fit, keys and splines.

A shrink fit mounted component is heated to expand the hub bore. It’s then placed on the shaft and allowed to cool.

A press fit hub bore is machined to exacting tolerances and then pressed onto the shaft.

A material fit typically means welding or soldering the mounted component to the shaft.

Although all of these are viable connections they are expensive, labor intensive, semi-permanent and by today’s standards somewhat archaic.

The “old standby” is the keyed connection, using a setscrew or quick detachable type bushing to hold the mounted component to the shaft. This also has its drawbacks.

Keyed connections need clearances between the component hub bore and shaft as well as between the key and keyway. As the torque is transmitted through the key, micro-movement occurs between the key and keyway. This can wear away small particles of metal that fill the clearance between the shaft and component hub bore, leading to fretting and corrosion. Once this happens, the mounted component is no longer easily removed.

Today, there is a need for precise, efficient running machines that maximize production while keeping material and fabrication costs to a minimum. The new answer is emerging: a keyless locking device that makes multiple connections possible, can lock almost anything to a shaft, is easily installed and easily removed.

How They Work:
All keyless locking devices use the same basic principle: creating a strong radial clamping force between the shaft and mounted component hub. This generates enough frictional force to prevent movement between the connected parts.

The most popular designs of keyless locks use an inner, collet-like sleeve with a tapered outside diameter and an outer sleeve with a tapered inside diameter.

The tapers are identical but opposite to one another. The inner sleeve fits around the shaft while the outer fits inside the hub bore of the component to be mounted. By tightening a single nut or multiple capscrews, the tapers are axially forced together causing the inner sleeve to clamp around the shaft and the outer sleeve to expand outward against the hub bore of the mounted component.

This frictional bond transmits torque, resists shock and torque reversals and eliminates backlash, key wallowing and fretting corrosion associated with a traditional keyed connection.

Advantages:
Keyless locking devices connect the mounted component to the shaft via a mechanical shrink fit, therefore eliminating the need for a key. Since a key is not required to transmit the torque, a keyless locking device allows a given shaft diameter to transmit greater torque and allows as shafts, bearings and mounted components to be smaller. This reduces size, weight and lowers costs. In addition the mounted component only requires a straight bore with a good surface finish obtained with common machining equipment, saving additional time and money.

Many machines have motion control, requiring accurate positioning, frequent stop-starts, and reversing loads. Frequently these machines are driven by a synchronous belt drive. The power source is often a servo or stepper type motor.

The keyed connection’s inherent backlash reduces machine accuracy which continues to deteriorate over time as the keyed connection wears. This loss of system accuracy requires component adjustment. With a keyed connection, this is time consuming and involves costly downtime.

A keyless locking device is a true “backlash free” connection eliminating the minute movement, fretting corrosion problems associated with a keyed connection and offers tremendous versatility in use. If the system does need adjustment the keyless locking device provides infinite radial and axial adjustment of the mounted component. Simply loosen the unit, make the necessary adjustment and retighten the unit, all in a matter of minutes.

Design Considerations:
To ensure the keyless locking device performs as required and provides a long term viable connection, there are several things you need to know about the application.

Shaft Size – Keyless locking devices are specified by the shaft diameter they are going to be mounted on. Most manufacturers offer them in both metric and imperial (inch) sizes.

Peak Torque – Accurately identify and quantify the applied peak dynamic load. Keyless locking devices have a known load-carrying capacity. If the rating of the keyless locking device is exceeded the connection will slip and continue to do so as long as the excessive load continues. To avoid this kind of failure it is paramount the application’s peak load is determined.

Shaft and Hub Bore Tolerances – Knowing accurate shaft and mounted component hub bore dimensions are essential in the selection process and must conform to the allotted tolerances specified by the manufacturer. A shaft that is under the allotted tolerance can reduce holding power and lead to potential problems.

Mounted Component – In addition to the mounted component hub diameter the yield strength (psi) of the material must be known. Keyless locking devices generate an outward pressure on the mounted component and the wall thickness must be sufficient to withstand these pressures or the mounted component will burst. Many manufacturers publish a formula or table that lets designers calculate the required wall thickness.

Possible Pitfalls:
When keyless devices do not perform as expected, the problems may be diagnosed:

Improper Installation – Probably the most common oversight. For the unit to perform as expected it, must be installed with a torque wrench and to the manufacturer’s recommendations. When working with a multiple screw unit it is imperative the capscrews be tightened evenly and progressively in a diametrical pattern to the required torque.

Diminished Clamping – Clamping capability diminishes as the shaft surface finish gets smoother. Keyless locking devices operate best and can transmit their full rated capacity when the shaft finish is from 32 -125 Ra. If turned, ground and polished shafting is used, its surface finish may be below a 32Ra. For some types of keyless locking devices this is too smooth and may require a roughening of the shaft to bring it up to acceptable shaft finish Ra levels.

Out of Tolerance – Manufacturers publish allowable component hub bore and shaft tolerances. A shaft or mounted component hub bore that is not within these published values will not allow the keyless locking device to fully clamp, resulting in movement between the connected parts.

 

SELECTION GUIDE:

Application or Condition Type of Device
Single Nut Multiple Capscrew
Shaft dia. 2″ and less

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