Keyless devices get a grip on shafts

Oct. 1, 2000
For reliable shaft connections, keyless locking devices may be the answer. These devices securely connect drive component hubs to shafts in demanding applications that require high torque or precise motion.

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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They also solve synchronization problems found in drive systems such as conveyors driven by dual chains. With keyed components, the keys and keyways transmitting torque to each chain eventually wear, and unless the wear is equal on both sides, the chains become misaligned. With keyless locking, however, a simple adjustment of the locking bolts eliminates the misalignment.

According to the purchasing manager for the Sugar Cane Grower’s Cooperative of Florida, keyless devices enhance efficiency because cooperative members can easily adjust them to keep the dual chains on cane harvesting equipment properly timed.

Any application with frequent starting, stopping, or load reversal is a candidate for keyless locking of hubs to shafts. Such applications impose impact shock loads on keys and keyways, or splines that may lead to wear and failure. Examples include overhead doors and cardboard forming machines. Many such machines, some dating back to the 1940s, are being retrofitted with keyless locking devices to correct such problems.

Machines that frequently transmit high torque, as in steel mills, are also good candidates for keyless locking. Gears and sprockets mounted on roller shafts must be securely locked so they can reliably transmit enough power to move tons of steel from one process to the next.

Selection and installation tips

In choosing a locking device, make sure that it matches the requirements of your application, including size, torque capacity, and mounting arrangement.

Because the locking force is uniform around the shaft, designers often use a light-weight hollow shaft, as long as it has adequate strength. Another option is using a solid shaft with a smaller diameter because it has no keyway.

The walls of a hollow shaft must be thick enough to withstand the inward pressure exerted by the locking device. Similarly, the hub of the connected drive component must have sufficient thickness to prevent it from deforming under outward pressure exerted by the locking device. Consult the manufacturer’s literature for such design information.

You can also use these devices in retrofit applications. Some types can be applied on shafts with previously used keyways. Other types, however, may require new shafts without keyways, and hubs that are rebored to remove the keyways.

Before installing a locking device that has tapered rings, lubricate the outer and inner ring surfaces that contact the hub and shaft, plus bolts, shaft, and drive component hub with a light oil film (never with molybdenum disulfide). Then, slip the locking device and drive component onto the shaft. Tighten the device by hand until you feel a slight positive contact. Then finish tightening with a torque wrench to the exact torque listed in manufacturer’s tables.

Where locking bolts or screws are positioned around the perimeter of the device, be sure to tighten them in the sequence specified by the manufacturer. Usually, this means tightening them as pairs located directly across from each other. Start by tightening the bolts to one-fourth the torque specified by the manufacturer. Then, re-tighten them to one-half the required torque. Next, using the same sequence, tighten them to the final torque. Finally, reapply the specified torque to ensure that the bolts will turn no further.

Kevin Powers is the manager, Power Transmission Components Div., U.S. Tsubaki Inc., Wheeling, Ill.

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