Avariety of locking devices is available to mount drive components to shafts. This article describes keyless locking devices, often called keyless bushings, that use friction to lock onto a shaft and to the hub of a drive component such as a sprocket, pulley, gear, timing cam, or roller. Other devices lock to a shaft by means of a key and keyway, as described in PTD, “Making the right shaft connections,” 8/96.

Many keyless devices work by means of mating tapered rings. Bolts draw these tapered rings together, exerting radial pressure that forces the rings outward against the hub and inward against the shaft. Friction between the tapered surfaces locks the drive component to the shaft, eliminating the need for keys and keyways, splined shafts, threads, and grooves. A keyless device applies locking pressure uniformly over a large area around a shaft circumference and inside the hub, enabling it to transmit higher torque and shock loads more consistently than one that relies on a key to transmit torque.

Because the shaft and drive component are locked firmly together, keyless locking devices prevent problems associated with keyed devices such as shaft play, slippage, backlash, and position misalignments. You can easily reposition keyless devices to adjust synchronizing or timing functions.

A keyless locking device generally costs more than a keyed device (except for large diameter or long shafts where keyway machining could cost more). But, its reliable, nonslip locking provides cost-effective solutions for applications involving high torque or sudden starts and stops.

These devices have been popular in Europe and Asia for many years, though U.S. acceptance has been slower. Standard units available in the U.S. accommodate shafts ranging from ¼ to 20-in. diam, depending on the manufacturer and type. Equivalent metric sizes are also available.

Stainless steel and polymer coated versions are useful in food processing and other damp environments where corrosion is a concern.

Basic types

Keyless locking devices come in many versions, including several types that use tapered components to apply locking forces against shaft and hub. Other types use expandable sleeves, pressurized fluid, or other means. Here are a few examples.

Double taper ring devices are probably the most common type, Figures 1 and 2. Each unit consists of two tapered rings, plus split inner and outer rings that mate with the tapered ring surfaces. An installer tightens bolts around the circumference of the device in sequence to draw the tapered rings together axially. This forces the inner and outer rings apart against the shaft and hub respectively, until friction holds them securely in place.

Single loading nut devices use a large threaded nut to draw a tapered outer sleeve against a tapered inner collet, Figure 3. Wedging action of the tapered surfaces squeezes the inner collet onto the shaft and expands the sleeve outward against the hub bore. Because this type requires tightening only one nut, it can be quickly installed, removed, or repositioned.

Threaded taper bushings use slotted inner and outer sleeves with matching tapered surfaces in the form of screw threads, Figure 4. A large nut on the inner sleeve contains a series of screws spaced around its perimeter so they butt against the outer sleeve. As the screws are tightened, they push the outer sleeve away and up the threaded taper, forcing the outer sleeve against the hub bore and the inner sleeve against the shaft.

Bellows sleeve devices expand concentrically outward and contract inward to grip the hub bore and shaft when axial bolts are tightened. Some models fit entirely within the bore, Figure 5, which makes them suitable for applications where axial space is limited.

Semifluid devices operate by fluid pressure. Each unit consists of a double-walled sleeve filled with a semifluid medium, Figure 6. One end of the sleeve is closed, and the other contains a piston, loading flange, and one or more screws. As the screws tighten against the flange, the piston pressurizes the fluid, causing the walls to expand against shaft and hub. This device can’t be used on shafts or hubs with keyways, flats, or splines.

Applications: precise to powerful

Because they prevent backlash, keyless devices are wellsuited for machines that must deliver precise motion. Examples include machine tools that call for close tolerances, complex packaging and bottling systems where timing is critical, and rotating equipment such as highspeed punch press turntables. Such turntables must precisely align parts time after time during extended production runs.

Keyless locking devices are useful in servo and step motor motion control applications because they eliminate clearance between worn components that could negate the precision provided by the motors.

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