Machine Design

Friction springs put a damper on things

Friction springs handle high forces in a relatively small package.

Friction springs act as buffers to absorb and dampen high levels of kinetic energy from a moving mass. They also work as overload-protection devices and handle high forces in a relatively small package. Certain drilling hammers use 1.14-in.-diameter friction springs with an end force of 2,470 lb, for example. Standard springs come in 0.75 to 16-in.-diameter with end forces to 400,000 lb with special configurations upon request. RING-springs require relatively little maintenance. Unless subjected to wash down or water-submerged situations, or unless there is very high ambient heat, friction springs can last for over 25 years. Special lubricants developed by Ringfeder boost lifetimes and dampening qualities.
A bidirectional spring cartridge.
A removed section reveals tapered inner and outer elements. A compressive axial force causes the elements to slide on one another in a wedging action.
Friction springs absorb recoil from Bushmaster guns in Bradley Fighting Vehicles.

Carl Fenstermacher
Ringfeder Corp.
Westwood, N.J.

Friction springs are candidates for any application involving absorbing impact. Their common uses: pneumatic hammers, steel mill/rolling mill end stops, railroad car coupler buffers, recoil systems in military cannon and automatic weapons, shock absorbers for heavy industrial doors, dock buffers for ferry boats, and buffers for crane end stops.

RING-spring friction springs from Ringfeder Corp., Westwood, N.J., contain a series of separate inner and outer rings with mating tapered faces. An axial compressive load causes the lubricated ring surfaces to slide axially against one another in a wedging action. Springs return to their preloaded condition when loads are removed. The arrangement absorbs up to two-thirds of an impact load as heat.

Reaching maximum spring travel causes inner-ring plane surfaces to touch and form a rigid column, preventing stresses from exceeding allowable limits. However, excessive loads should be avoided because friction springs don't provide damping when in a blocked (solid) position. When blocked, high peak forces transmit to and could compromise attached components. One half of one inner and outer ring makes one element. Spring travel and stiffness is proportional to spring-element count.

Conventional springs (leaf, coil, volute) develop maximum stresses at outer edges of a section while cores remain unstressed. Friction springs, in contrast, have a more uniform stress distribution. Tensile stresses in outer rings and compressive stresses in inner rings are nearly constant. Fully utilizing spring material in this way helps shrink device size and weight.

Friction springs work over a temperature range of -20 to 60°C without appreciable changes to force-deflection curves. Damping action is independent of load-application rate though spring heating should always be considered. Conventional fluid-type dampers, on the other hand, don't work well when loads are applied slowly. Moreover, temperature fluctuations and inherent temperature rises change force-travel qualities of both fluid-type and elastomer dampers.

In any case, spring work is the key metric. Aim for the smallest possible force in end positions to protect structural components. This is contrary to conventional approaches that say buffers must produce a high terminal force. Buffers should absorb as much energy as possible and produce low terminal forces for maximum effectiveness.

Make contact:
Ringfeder Corp.
(888) 746-4333

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