Lock it up

Moving equipment at high speeds requires stability. The use of shaft-hub locking devices dampens vibration while transmitting high torque. To help us understand how locking devices work, we asked the experts to explain what makes them suitable for high speeds, and how to get the most out of their use in motion control systems. Read on to learn how to most effectively translate axial loads into radial loads with these components.

Which locking device attributes are most closely linked to speed, and how do they affect it?

Mark • B-Loc: There are three benefits to keyless locking devices for high-speed applications. First, eliminating keys and keyways means shafting and drive components have vastly improved balance characteristics. Second, they are precision machined with shallow mating tapers to provide excellent concentricity with through-bored components. Finally, when compared to keyed bushing systems like QD or taper-lock, keyless locking devices offer a reduced cross section, which translates into less mass and lower inertia.

Andrew • Whittet-Higgins: Restraint and balance are the most tightly linked locking device attributes; however, the causal aspects of speed that affect these attributes should be noted. These include faster acceleration, faster deceleration, and operating speeds at tens of thousands of revolutions per minute — speeds that exert increased forces and stresses throughout machine assemblies. To optimize the performance of speedier machine and equipment assemblies, better component-to-component contact, better thread formations and tolerances, tighter thread-to-face tolerances, dynamic balance, and positive mechanical locking have increased in use.

Andrew • KTR: Keyless shaft-hub clamping devices eliminate the need for keys, keyways, or spines and are designed to frictionally connect the drive shaft to a driven component by inducing an interference fit. The locking device can even grasp undersized or oversized shafts/hub diameters and provide a strong connection at a reduced torque rating, avoiding a loose “keyed” component that can cause vibration and even injury at running speeds. The vibration or imbalance caused by a loose fit can be detrimental to the machinery, causing undesirable resonance or torsional vibration.

Many locking devices come in self-centering and non-self centering designs. For non-self centering locking devices, the concentricity of the hub towards the shaft depends on the fit and length of the hub pilot. The symmetrical and concentric design of many locking devices allows faster speeds than other conventional locking methods such as keys, clamping hubs, or double setscrews. This is due to the internal design, proportional weight distribution, and concentricity between the shaft and hub.

Bob • Ringfeder: Locking devices create a mechanical shrink fit. Locking screws translate axial clamp forces, through tapered rings, into a contact pressure that creates an interference fit.

Internal locking devices expand into the bore of a component and contract around the shaft. External locking devices fit over the hub and squeeze it around the shaft. Expansion due to centrifugal force will cause a loss of fit in both types.

It has been determined that peripheral speeds greater than 2,000 in./sec at the mounted component's largest diameter will reduce the interference fit produced by the locking device. This proportionally reduces the torque carrying capability of the connection.

For internal devices the hub outside diameter must be used in calculating the effect of centrifugal force. For external locking devices the outside diameter of the device is affected. Calculations should be run to determine if the speed is too high for the connection to remain viable.

What are some of the limiting factors associated with locking devices in terms of speed, and how are they overcome?

Andrew • KTR: Many types of locking devices cause internal stress in the driven components. The stress increases when speed is added to the equation. It is important to know the parameters of the application and what is being coupled by the locking devices to evaluate and ensure the integrity of the setup. If the locking device is used on a component with thin walled sections, stress calculations must be conducted so that a suitable locking device is chosen.

When a mass coupled by a locking device is accelerated or decelerated to a certain speed it is always wise to calculate the torque from the inertia. The locking devices can slip either on the shaft or hub when the transmittable torque plus the inertia torque exceed the locking device's capacity. Understanding the manufacturer's installation and assembly guidelines is critical to the ultimate performance of the application. Caution must be taken not to jam or tilt the device as it is being tightened. A poorly assembled locking device can create severe repercussions at running speeds.

Bob • Ringfeder: Internal locking devices lose fit due to expansion of the hub. The contact pressure produced by the locking assembly must be greater than the loss of pressure due to centrifugal expansion. The strength of the hub material is very important. Internal locking devices create stresses in the hub and centrifugal force will add to that stress. Materials must have a yield point that is greater than the combined stress. Hub geometry is also important. The contact pressure produced by the locking device can cause distortion in the hub that would be difficult to remove by balancing.

When you compress the hub onto the shaft, only the outer ring outside diameter is affected by centrifugal force, which allows a larger diameter component to spin at higher speeds. Compressive stresses created by the locking devices act in the opposite direction from the stresses due to centrifugal force: This reduces the need for higher strength materials and minimizes distortion.

Andrew • Whittet-Higgins: The major limiting factors for failure of assemblies at high speeds are unbalanced components, unbalanced assemblies in design as well as dynamic performance, elevated temperatures and environments in which components must perform, and dependence on thread contact locking alone.

To overcome these obstacles, locking devices can be designed and manufactured with tighter thread tolerancing. This improves thread-to-face runout and minimizes damaging movement between components in the assembly, by dynamically balancing high-speed applications. An unbalanced assembly can loosen or fracture itself, damaging the components, equipment, and even at times the people near it. In instances where sudden dynamic loading of an assembly is needed, hardened alloy material reduces locking device failure and protects equipment assemblies.

Mark • B-Loc: For external clamping-type keyless locking devices, centrifugal forces from excessive speeds can erode frictional fit pressure. In some cases, this could result in a failure of the connection. Further, keyless locking devices are typically supplied with a cylindrical friction surface that is slit lengthwise for flexibility. However, in certain extreme high-speed environments the resulting marginal imbalance can be problematic. In these cases the keyless locking device design is modified: Machining tolerances and fit clearances are tightened and the friction surface remains unslit.

What's the highest speed application that your locking devices have gone into, and what special considerations were necessary to make it possible?

Andrew • KTR: Certain high-speed applications require well-balanced components to operate smoothly and avoid unnecessary vibrations. Some of our clamping rings achieve speeds to 40,000 rpm. These have been successful in a high-speed spindle application that required both precision machining and two-plane dynamic balancing. Careful machining and inspection ensured that the allowable concentricities and runouts were met. Every application has its limitations and pushing those limits to supply an economical solution — without sacrificing performance — is a continuous give and take.

Andrew • Whittet-Higgins: One of our most difficult applications came from a special equipment builder who was constructing a spindle that would run at a maximum speed of 80,000 rev/min. The difficulty of the application wasn't only the maximum speed, but the acceleration rate at which the machine was to perform. To rotate from zero to 80,000 rev in a few seconds creates a tremendous shock load on every component in the assembly, including the retaining device. To counteract these forces we proposed a modified version of one of our balanced and hardened retaining devices. A normal Class-3 thread was augmented with more controlled radii on root and crest to limit flank differentiation, especially at pitch depth. We took the tolerance between the shaft thread and locking device thread to nearly zero, and the face-to-thread runout to under 0.0001 in. Additionally, the customer elected to use a generous amount of thread locking compound as a safeguard.

Bob • Ringfeder: Internal locking devices have been used in 3,000 to 4,000-rpm applications. Usually the complete assembly must be balanced after the locking device has been installed.

Our three-piece designs have been used up to 7,000 rpm. Careful installation is required to insure that the outer rings are kept parallel. Assembly balancing is not always necessary; balancing the locking unit alone on an arbor is usually sufficient. Too, the component separately allows assembly without the need for match marking.

A two-piece design was designed for test bench applications up to 15,000 rpm.

Mark • B-Loc: We frequently specify keyless locking devices for applications with speeds of 10,000 rpm and higher. Primary considerations include those mentioned previously — inertia, concentricity, and balance requirements — as well as those associated with all keyless locking device applications, such as torque and overhung load requirements, alignment, and timing capabilities.

For more information, contact the editor at [email protected].