TechArchive

Original article date: July 1998

Adhering to a few simple design considerations for locking assemblies make them so much more effective. Alan Quinn studies the design advice given by Simplatroll for the Tollok range.

Applications for locking assemblies are found everywhere from simple pulley fixing to heavy duty drums and high integrity cases such as lift pulleys. V-belt timing chain and other types of pulleys are mounted in vast numbers using locking assemblies which are also attractive for independent setting for cam timing without the expense of keyways. They can be used on any hub which is normally shrunk keyed splined or pinned to a shaft – for example flywheels or drive drums.

The Tollok range of locking assemblies available from Simplatroll can accept high axial loads in static and dynamic applications whilst transmitting torque. In use it is every important to ensure that both hub bore and shaft diameter are within the stated tolerances. If either are out of tolerance there could be a reduction in the transmissable torque since more of the force created by the bolts is used to close the gap between the locking assembly and the shaft or hub.

No special surface finish is required – a rough ground or fine-turned finish is ideal. The peak-to-trough height of the surface must be less than 16um. In many machine applications such as bearings and gears a low coefficient of friction is desirable. But a locking assembly relies on friction to transmit the torque so a predictable and repeatable coefficient of friction is critical. Table 1 shows the typical values to aim for:

The majority of applications have solid steel shafts and steel hubs although hollow shafts require special attention and a formula exists to calculate the maximum permissible shaft bore. Tightening the locking assembly screws creates surface pressure on the full contact area between the locking assembly and the hub and between the locking assembly and the shaft. Shaft values are normally higher than hub values but are generally only relevant is the shaft is hollow. Low values allow thinner walled hubs. The locking assembly must have a hub surface pressure lower than the yield point of the hub material or failure will occur. This is especially relevant with aluminium and cast iron.

When using cast iron as a hub extreme care must be taken not to overstress the iron. Cast iron fractures rather than yields and a safety factor must be employed to protect it. Aluminium is becoming increasingly popular due to the need for low weight low inertia pulleys for servo applications and for environments where steels would quickly corrode. When considering aluminium hubs the low yield point generally forces the selection of a large hub. Certain series of Tollok locking assemblies particularly TLK110 have very low surface pressures and are therefore suited to aluminium hubs.

High grade aluminium alloys (Durals) do not yield in the same way as steels. Generally fracture occurs on a similar way to cast iron but they have very high yield points so this does not pose too much of a problem. The main concern is where a very small hub is used.

Where the locking assembly is not self-centring a centring guide is necessary to avoid unpredictable levels of axial and radial runout. The centring guide should be at least as long as the clamping surface of the locking assembly.

Irrespective of the application all hubs are subjected to internal pressure. The ability to resist this internal pressure requires a minimum hub diameter. Lower values will result in loss of torque and damage to the hub.

Bending moments arise from radial loads that are not balanced to the centreline of the locking assembly. For the vast majority of applications they are unimportant. However on some applications such as conveyor drums they do have to be taken into account. The result of bending moments is a localised increased surface pressure on both hub and shaft. This in turn reduces torque transmission.

Values depend on the design of the drum diaphragm. Flexible construction of the diaphragm keeps the surface pressure increase to a minimum. However generally bending moment data is calculated on the worst case of a rigid diaphragm.

During fitting hub shaft and locking assembly contact surfaces should be cleaned with solvent to remove all traces of oil grease and anti-rust coatings. Screw threads and conical surfaces should be lightly oiled. For all locking assemblies except shrink discs molybdenum disulfide or other high pressure additives should not be used although they essential of the rated torque of the shrink disc ranges TLK 603 and 602 is to be achieved. Good advice is to tighten the screws first by hand then diametrically with a torque wrench to the specified value. It is essential to repeat the tightening process until the torque values are stable.

Simplatroll has introduced a guide for designers to the range of Tollok mechanical locking assemblies. It describes the 15 types available for shaft sizes from 6 to 900mm together with a selection flowchart to guide the user through the multiple factors that influence the selection of the optimum locking assembly.

• Simplatroll
• 01234 350044
• Geoff Spear

July 1998