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Locking assemblies

In mechanical engineering, locking assemblies are a shaft-hub connection. This is non-positive and unites a hub (e.g. of a sprocket) with a shaft and the machine element. Machine elements can also be feather keys. This creates a torque between the hub and the shaft. The hub is secured on the shaft (axial displacement).

How locking assemblies work

These are available in different designs depending on the intended use. The clamping elements often include two annular spring-shaped rings. These are slid over each other on tapered surfaces. Both the outer and inner surfaces of the clamping sets are cylindrical. These are inserted into the bore of the hub (clearance). Several screws with axial alignment ensure that the tapered surfaces of the outer and inner ring can be tightened for clamping. The taper surfaces can also be divided into several parts in a sawtooth shape. These are either turned from one piece or several pieces.

The taper surfaces (axial preload force) cause the outer ring to widen and the inner ring to shrink. This creates a pressure on the shaft. In some variants, slotted outer and inner rings are used to facilitate the process. This creates a very high friction on the shaft and hub. This results in a tight connection. This means that torques can be transmitted very well. If the taper angles are rather flat, self-locking occurs. It is then no longer possible to open the clamping sets.

One of the possible designs uses various shrink discs. This is placed on a hollow shaft or hub from the outside. It then generates pressure on the hollow shaft by reducing its internal diameter. This is done by means of tapered pressing surfaces of the outer parts. An interference fit is thus produced. This is a completely backlash-free connection between hub and shaft. The necessary preload is applied hydraulically or by high-strength screws.

Where are locking assemblies used and what are the advantages and disadvantages?

For installation, locking assemblies must have a cylindrical bore on the hub and a cylindrical end on the shaft. These can be manufactured simply and are easy to install. This eliminates the need to manufacture complicated grooves or profiles, which is a major advantage. The only disadvantage is the greater space required for installation compared to material and form-fit connections. The prerequisites for installation include suitable dimensions of the clamping set and sufficient wall thickness of the hub. If there is not enough space for the Allen key (assembly tool), radially mountable models are available (special key). For shaft diameters, clamping sets are available in a very wide range. The standard range is between 5 mm and 1,000 mm.

Due to the good concentricity properties, clamping sets can be used very well for precision work, such as in machine tools. With appropriate design and constructional measures, clamping sets can be used as overload protection for various machine parts.

Further advantages of locking assemblies:

The play between the driven element and the shaft is eliminated. The load is distributed evenly over the entire diameter. It is relatively easy to synchronize organs connected to each other. The stress cross-section of the drive shaft is increased. The assembly of the machines is quick. Machine downtimes are eliminated due to simplified maintenance. Spare parts from different manufacturers can be used together.

Special shapes

Instead of the axis-parallel clamping screws, the outer and inner rings can be screwed together (quick-action locking assemblies). In addition, there are diaphragm-shaped clamping sets. Taper bushings function like tapered clamping sets. These get by with one ring. However, the hub surface must be tapered.

Tips for handling locking assemblies

Force-fit connections are only as secure as the calculated and generated pressure. In addition, the stress on the components and the permissible limit values must be kept in mind. It makes little sense to install parts that do not fit exactly, are already slightly damaged or are not suitable for this load. If product-specific conditions are taken into account, clamping sets can also be installed where it was previously not possible (bending stress).