Motion System Design

Friction springs minimize impact, vibration

Friction springs consist of radial rings that deform briefly and convert impact energy into frictional heat. Design them into your next design for damping.

Spring technology for damping is relatively old. Stagecoaches had leaf springs, early trains had large, crude, coil-spring buffers (to absorb shocks when cars bumped together) and early automobiles made use of simple springs for damping as well. However, these early springs were roughly assembled and packaged, so they generated inconsistent force and unpredictable behavior. Then a tapered ring system was developed: These friction springs (as they are now called) were originally developed for use as primary railway buffers. Engineers stacked these tapered rings and found that they could absorb shock in a fairly controlled manner. Today, they’re used in a myriad of applications.

How they work

Friction spring dampers consist of a series of concentrically stacked steel rings with mating taper faces. A stack can include two or 200 rings, depending on how much force and travel the design must absorb. Inner and outer rings alternate down the length of the friction spring; under application of axial load, the wedge action of the taper faces expands the outer rings, contracts the inner rings radially, and allows axial de ection.

When compressed, each solid ring slides on a taper, and these in turn rub against each other and generate heat. For example: If we apply 1,000 lb to a spring, it absorbs about 66% of that force as heat. The material deforms, but compression is within the elastic material limit, so deformation is temporary.

There is recoil — in industrial systems, thousands or tens of thousands of pounds. However, with friction springs, designs can absorb the impact of big press hammers, jack hammers used in construction, or even cannons, so shock load is not transmitted through frames or structures, and cannot hurt anyone.

During manufacture, the rings are cut from spring steel, rolled or turned, calibrated for size, hardened, and tempered. The rings are then given a nal dimensional check. Friction springs can absorb energies from 1 J to more than 100 kJ. For overload protection, absorption of impact energies is 0.1 kN to 10,000 kN. Friction springs make the most of the material from which they are made — so their dimensions and weight are minimal.

During operation, friction springs require lubrication. Speci cally, engineered greases are necessary to withstand the shearing friction of tapered ring operation without the degredation or migration that other greases can exhibit. Manufacturer-engineered grease can be tailored to ne tune damping output force generated by friction springs.

Damping technology comparison

Most engineering programs don’t cover friction springs, so many U.S. engineers aren’t fully aware of their valuable capabilities. Even so, their use grows. Friciton springs are replacing coil springs and hydraulic buffers in many applications because these purely mechanical tapered springs require little or zero maintenance, and operate for decades with no problems.

Unless they are overloaded, friction spring rings in most applications do not wear out. German and some Canadian railroad lines remove their springs every 15 or so years, inspect them, relubricate them, and put them back into service. In contrast, friction springs in jackhammers and some machine-gun applications do wear out — so after a certain number of cycles, they must be replaced. However, to reiterate, friction springs in most applications are reused indefinitely.

In comparison, more complicated hydraulic dampers can be tuned to give more or less damping, but rely on hydraulic uid to control their function. So, they’re subject to leaking, require maintenance, and (in steel mills, for example) are vulnerable to heat due to their rubber seals and potentially ammable oil. In addition, hydraulic dampers and springs of synthetic material are in uenced by temperature uctuations and inherent temperature rises.

In contrast, the characteristic curve of a friction spring remains independent of these factors within certain limits. Friction springs can be employed in the temperature range of -40° to +180°C without much change in force capacity. Here, allowances must be made for the inherent temperature climb caused by damping. Note: For extreme applications going beyond indicated temperature ranges, consult the manufacturer; mounting options can often address heat issues. What’s more, friction spring nesting geometry allows optimization of available mounting space with parallel and series spring arrangements.

Typical applications

One place you won’t nd friction springs is at the bottom of an elevator shaft. Instead, there are usually several long, fat coil springs that don’t dampen much. Why? Simple overload protection such as this requires only simple safety devices. In contrast, if a design must protect goods or personnel, it requires more damping.

To illustrate, friction springs are used on tools that machine steel. On tool holders, if there is a jam or the holder impacts a rotating machine part, friction springs absorb the shock, and can even cause the machine to shut down, so that an operator can investigate. Similarly, in drilling operations (to obtain water or oil, or to cut away at a mountain, for example) friction springs protect equipment from impact when hard or uneven material is struck. Friction springs also reduce the shock level experienced by jackhammer operators. In bay doors of European cargo planes, friction springs dampen any impact upon closing as well.

Another use: Blocking

In contrast to velocity-dependent spring systems, friction springs provide full spring work and damping even when load is applied quickly or very slowly. Friction springs also block load, so when maximum spring travel is reached, the front surfaces of the inner rings touch and form a rigid column. For this reason, admissible stresses cannot be exceeded and friction springs suffer no damage. Even so, in actual applications such loading conditions should be avoided because the spring is not effective in its “blocked” (or solid) position: High peak forces may result and jeopardize structural components.

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