Locking devices: basic designs and applications

shaft-collar-close-up

This one-piece shaft collar works as a spring tensioner.

A locking device is a mechanical component that prevents mated shafts and other machine elements from moving out of position when subjected to external forces. Operating conditions such as initial installation error, temperature variations, vibration and others can all cause issues. These are critical components. The safety of an entire system often relies on locking devices. They are common in systems that require coupling multiple components.

Designers use shaft collars in myriad moving machinery applications—including designs for aerospace, mechanical, medical, and industrial industries. In electric- motor-driven designs, they’re most common at the gearbox and motor assemblies. Shaft collars accomplish 3 basic functions:
• set shaft position
• space components on shafts
• limit shaft movement

mechanical-stop

One-piece shaft collars used as a mechanical stop to control the stroke of a linear slide.

Shaft collars often act as mechanical stops on cylinders and actuators, locating elements for motors and gearboxes, and for keeping shafts connected with bearings and sprockets. Some shaft-collar variations are more suitable for given applications than others.

Setscrew shaft collars are low cost with easy installation. As such they quite common regardless of the fact that clamping collars have been around for some time. Setscrew shaft collars are still common in today’s applications that don’t need post-installation adjustments and where cost is a concern.

CLAMP-STYLE SHAFT COLLARS

Clamp-style collars use compression to affix components to shafts without marring them. These collars distribute clamping force uniformly around the shaft (and not at one point) so users can adjust them without shaft damage. Clamp-style shaft collars are either one-piece or two-piece collars; the two-piece collar most evenly distributes force and creates the strongest hold. While these types of collars are found in nearly any industry where power transmissions are present, they are increasing in medical applications.

Like other shaft collar styles, clamp-style shaft collars work as guides, spacers, and stops on medical equipment. Only collars with tightly controlled face-to-bore perpendicularity, however, (with a TIR of less than or equal to 0.05 mm) meet the design specifications in certain critical designs—specifically where the collar functions as a load-bearing face or aligns critical motion components such as bearings or gears. Two-piece collars have stronger holding power than one-piece designs and install in place.

sprocket-on-shaft

One- and two-piece shaft collars use to align a
sprocket on a shaft.

Some shaft collars use fastening hardware that exceeds industry standards for maximum torque capabilities and holding power. Some stainless-steel shaft collars have even hardware that can also resist corrosion and satisfy regulatory standards. The stainless hardware undergoes surface treatment to prevent galling. Other clamp-style shaft collars come in steel with a zinc or black oxide finish, aluminum, titanium, and plastic. Black-oxide on steel resists corrosion and smooths screw installation to boost the transfer of screw clamping forces to the collar-shaft interface while preventing stick-slip.

Frictional locking devices leverage a coefficient of friction between contacting surfaces. So when installing these locking devices, internal elements expand to fill the gap between machine shaft and hub— to keep components in place with friction. These devices usually take the form of metallic or non-metallic hollow cylinders, often with a slit on one side. ANother friction locking device is the ubiquitous nut — using friction on shaft threads and slight bolt tension and compression of parts held together for secure attachment.

Frictional locking devices don’t require keying, so aligning keys and key ways is unnecessary… and requirements to match geometry are forgiving— so these often accommodate over the undersized shafts.

Keyless designs also avoid loosely keyed components and their reduction in safety, accuracy, stability, and torque transmission.

End users assembling frictional locking devices into a design simply insert the shaft into the locking device… and then the device exerts radial pressure to lock components in place. The backlash-free operation (assuming a proper fit and tolerances) allow for precise adjustments to axial position and angular timing in a system… and (in the case of axes that need to make reversals) there’s no impact between key and keyway.

These design elements make friction locking devices a viable option for many applications. With their compatibility and ease of use, engineers often select them for a variety of situations. But which situations are best suited for frictional locking devices and which are best to avoid? Generally, engineers should avoid specifying them in systems with high external centrifugal forces. These situations can cause a drop in the pressure between the components and lead to slipping. Because there is often a small slit in frictional locking devices—to accommodate shafts of varying diameters—these can cause imbalances in certain operating conditions, usually at higher speeds. In such applications, engineers can use slit-less friction locking devices, which have stricter machining and application tolerances or use another type of locking device.

Smaller frictional locking devices work where lower torque is needed. Systems delivering high torques may need larger variations or even specialty configurations. Engineers should consult manufacturer documentation to ensure their design calculations are accurate. Manufacturers usually provide instructions for sizing locking devices but offer assistance for specialty applications.

Custom-manufactured shaft collars can have modified slots, flats, through-holes, and threaded holes… as well as cams, hinges, mounting holes, and more. These features often reduce the number of parts needed in a system and improve structural integrity mechanical efficiency, and drive performance.

Speak Your Mind

*