During design, engineers must decide how to connect each involved power transmission component (and other items) to shafting. Traditional connection methods fall into the three categories mentioned. A shrink fit requires the mounted component be heated to expand its hub bore; then it's placed on the shaft and allowed to cool. A press fit requires the hub bore be machined to exacting tolerances, so it stays put when pressed onto shafting. A material fit typically entails the welding or soldering of the mounted component to the shaft. These are viable connections, but they can be expensive, labor intensive, semipermanent, and somewhat archaic.
The old standby is a keyed connection. With this mounting, a setscrew or quickly detachable bushing holds components to shafts. To make installation possible, clearances must exist between the component hub bore and shaft, as well as between key and keyway. These industry-standard keyed connections have a number of limitations. For one, the process of cutting a keyway is time-consuming and permanent. (Once cut into a gear or other part, a keyway can't be filled back on.) Mass production offsets some cost, but the resources to mill or broach a keyway, machine tapers, and drill and tap holes into mounting components are expensive. The keyway also weakens the shaft and reduces its torque-transmission capabilities.
The inherent backlash in keyed connections reduces machine accuracy that continues to deteriorate over time. With the loss of system accuracy, components may need to be adjusted to their original setting … and with keyed connections this process can be time-consuming.
Torque transmitted through the key induces micromovement between it and the keyway. This causes small metal particles that fill the clearance between the shaft and component hub bore to wear away — in turn, leading to fretting and corrosion. Components can no longer be removed easily.
The global marketplace demands precise, efficient machines that maximize production time while minimizing material and fabrication costs. Unlike the other setups, clamping locking devices make multiple connections possible, and can lock almost anything to a shaft. Even though it's a mature technology, these keyless locking devices are still increasing in use as more designers acknowledge its versatility.
All keyless locking devices use the same basic principle: They create a strong radial clamping force between shaft and mounted component hub. If strong enough, this generates enough frictional force to prevent movement between the connected parts. If movement takes place between these connected parts, damage usually occurs in the form of fretting, galling, or scoring, which can result in poor performance and disassembly problems.
Although there are varying designs, the most popular employ a two-piece setup: an inner, collet-like sleeve with a tapered outside diameter, and an outer sleeve with a tapered inside diameter. The inner sleeve fits around the shaft while the mirroring outer fits inside the hub bore of the component to be mounted — whether that's a timing pulley, chain sprocket, gear, or other component that needs securing. Tightening a single nut or multiple capscrews, the tapers are axially forced together. This causes the inner sleeve to clamp around the shaft, and the outer sleeve to expand against the mounted component's hub bore. This frictional bond, similar to a mechanical shrink fit, transmits torque, resists shock, and endures torque reversals; it also eliminates backlash, key wallowing, and the fretting corrosion associated with traditional connections.
Keyless locking devices connect mounted components to shafts via a mechanical shrink fit. With no area weakened by a key, keyless locking devices permit a given shaft diameter to transmit greater torque … allowing system parts such as shafts, bearings and mounted components to be smaller. This reduces size and weight, which in turn lowers cost. Additionally, the mounted component only requires a straight bore and average surface finish, obtained with common machining equipment. In fact, when using keyless devices, more refined surfaces might be better avoided. We'll discuss this point shortly.
Many machines today perform some type of motion control that entails accurate positioning, frequent stop-starts, and reversing loads. Often these machines are driven by synchronous belt drives chosen for positive, precise, and efficient power transmission. The power source is usually a servo or stepper-type motor.
If a system needs adjustment, keyless locking devices provide infinite radial and axial adjustment of the mounted component. In minutes, an end user can loosen the unit, make necessary adjustments, and retighten the component.
Formulas exist for calculating clamping force, load capacities, and hub pressures. However, many manufacturers have eliminated the need to calculate requirements with published catalogs and websites that list all necessary data to correctly select a keyless locking device. For unusual or especially challenging applications, most manufacturers welcome the opportunity to participate in the selection process; designers are encouraged to utilize their expertise. However, for a keyless locking device to perform as required there are several values that must be defined beforehand.
Keyless locking devices are specified by the shaft diameter on which they'll be mounted. Most manufacturers offer them in both metric and English sizes.
The applied peak dynamic loads must be accurately identified and quantified. Keyless locking devices have a known load-carrying capacity. If the keyless locking device exceeds its rating, the connection slips — and does so as long as the excessive load persists. To avoid this kind of failure it is paramount to determine the application's peak load.
Shaft and hub bore tolerances
Accurate shaft and mounted component hub bore dimensions are essential when selecting connections. They must conform to the allotted tolerances specified by manufacturers. A shaft under the allotted tolerance can reduce holding power and lead to potential problems.
In addition to the mounted component hub diameter, the yield strength of the material (expressed in psi) must be known. Keyless locking devices generate an outward pressure on the mounted component, and its wall thickness must be sufficient to withstand these pressures. Otherwise, mounted components can burst. Published yield-strength formulas and tables are indispensable when calculating required wall thickness.
In addition to these basic parameters, designers should also review the application for space requirements, operational environment, number of assemblies/disassemblies, ease of installation and axial movement. Some or all of these can influence the keyless locking device selected.
When properly designed and applied, keyless locking devices can provide years of reliable performance. When they don't perform as expected, their failure is often caused by one or more of the issues we'll now discuss.
Improper installation is probably the most common oversight. For units to perform as expected they must be installed with a torque wrench and to the manufacturer's recommendations. When working with multi-screw units, it's imperative that capscrews be tightened evenly and progressively — in a diametrical pattern — to the required torque.
Although keyless locking devices exhibit excellent holding power, clamping capability diminishes as the shaft surface finish gets smoother. Typically most keyless locking devices operate best (transmitting their full rated capacity) when the shaft finish is between 32 and 125 Ra. If turned/ground/polished shafting is used, its surface finish may be below a 32 Ra. For some types of keyless locking devices this is too smooth and may require roughening the shaft to bring it back up to acceptable shaft finish Ra levels.
There is another possible pitfall using keyless devices. Users understand the principle — a tapered inner sleeve clamps the shaft and the outer sleeve expands against the mounted component hub bore — but they believe this range of expansion and contraction is rather liberal. For this reason, shaft size and component hub bore are sometimes overlooked. Manufacturers publish allowable component hub bore and shaft tolerances. In many cases, a shaft or mounted component hub bore not within these values prevents the keyless locking device from fully clamping … resulting in movement between the connected parts.
For more information, contact Fenner Drives USA at (800) 243-3374.
Heavy metal cranks it up
When the fighting robot named Heavy Metal Noise was developed for competition on television's BattleBots program, the industry-mainstay keyed connection was used to mount the robot's weapons to their shafts. However, problems soon arose with its twin aluminum “discs of death.”
Shock loads are produced when the weapons — rotating at very high rpm — strike opponents' robots. On several occasions during its first season, mounting hubs cracked; the keyways wallowed and eventually stress on the shaft keyways caused them to fail. Basically, one side would always shatter out. In one particular competition, one of the discs had to be removed. Part of the shaft was amputated and the robot continued its remaining competition with only 50% of the weapon system intact.
During the off-season while making repairs and improvements to Heavy Metal Noise, the robot's designers explored alternative connections. A material fit connection (which employs welding and a press fit) was possible. However, the team decided they wanted the ability to easily remove the weapon discs if necessary. Then they purchased a keyless single-nut design for mounting connections.
With the new season of BattleBots, the team was apprehensive about its performance. For this reason, they verified the connection's performance by match-marking the device, shaft, and mating hub on the weapon discs. Following the first combat, these marks were examined. There was no visible sign of movement between keyless locking device, shaft, or mating hub. The keyless locking device successfully withstood the shock loads from a 23-lb spinning weapon system (rotating in excess of 2,000 rpm) and repeatedly striking a 120-lb steel-armored opponent. After numerous battles, keyless locking devices still keep the weapons securely attached and able to withstand shock loads.
When to use
Following are some general guidelines when considering what type of keyless locking device to use in an application. Note that each type should be considered on its individual merits when determining its suitability for an application.
Better biking beat
One motorcycle drag-racing team optimizes their bike's performance with keyless locking devices — between races.
Picture a motorcycle with a nitro-fueled 1,100-hp supercharged engine. An 8-mm synchronous belt drive is the power transmission medium for its blower, fuel pump, oil pump, and magnetos. The Kutzera Top Fuel Yamaha Racing Team, Disputanata, Va., races just such a bike in regular drag races. Sometimes it's necessary to change the timing on the motorcycle. Keyless locking devices connect the sprockets to the respective shafts to drive and time dual magnetos. They make optimizing the dual ignition system timing quick and easy. And because keyless locking devices have infinite radial positioning capabilities, the crew can quickly vary the timing between 40° and 50°. Here are the steps involved:
- Switch on the bike's electrical system.
- Loosen the keyless locking device connecting the sprocket to the magnet and shaft.
- Rotate the synchronous belt drive until a continuous audible sound indicates the timing target has been achieved.
- Retighten the keyless locking device.
- The same procedure is repeated for the second magneto.
Wedging clamping devices offer another benefit as well. The engine on this high-powered motorcycle revs from 2,800-rpm idle to a top end of 10,500 rpm in one sec, moving the bike down the quarter-mile track in about six sec at 240 mph. Keyless locking devices withstand this initial shock and transmit the applied torque.