Machine Design

Shock Absorbers

A number of different shock absorber designs are available, but the fundamental operation of each is the same. Shocks are basically multiple-chambered cylinders, with one or more orifices between chambers. As an object strikes the cylinder rod, an internal piston moves, increasing fluid pressure in the cylinder. Fluid flows through the orifices, dropping in pressure and increasing in temperature. Thus, the kinetic energy of a moving object is converted to heat as it is stopped.

Efficiency and effectiveness of the absorber depend almost entirely on the leakage path between the two sides of the cylinder. But energy-absorbing capacity depends on the size of the shock absorber and the method of returning the piston to its rest position. Spring-return shock absorbers are more compact and convenient than external-accumulator models, but do not have as much energy capacity. Accumulator shock absorbers have more hydraulic fluid and more surface area from which to radiate heat. Therefore, they can be cycled more frequently at maximum capacity than spring-return models.

Hydroshocks are nonadjustable, multiple-orifice devices, with holes spaced along the length of a cylinder. When a hydroshock is loaded, an internal piston moves along the cylinder, closing the holes one at a time and decreasing the effective orifice area. Orifice size and spacing are critical and best accomplished by sophisticated computer modeling. Otherwise, proper orificing could take several months of hand calculations and testing.

The hydroshocks main advantage is its nearly perfect deceleration while its chief advantage is that it only works for one weight, velocity, and propelling force. If the shock is not properly sized for the application, the result is high collision or set-down forces.

Adjustable shock absorbers feature a series of orifice holes machined along the length of a fixed metal tube. A slotted metering tube, which fits over the stationary tube, can be rotated via an external ring to adjust total effective area and desired deceleration rate. When the metering tube is rotated toward the open position, the shock provides maximum orifice area and minimum resistance. Conversely, movement toward the closed position reduces orifice area and increases resistance. This adjustment method gives the capability to handle large weights or high propelling forces at low viscosities.

Adjustable shocks overcome the chief disadvantage of the hydroshock by adjusting the orificing to custom fit any input conditions. Therefore, a properly adjusted shock can produce the same nearly perfect deceleration as a hydroshock.

The shock's main advantage is the ability to handle a wide range of input conditions; it's chief disadvantage is that it must be manually adjusted each time the input condition changes.

Self-compensating shocks are fixed-orifice devices which require no adjustment, and cover a portion of an adjustable model's weight range. Self-compensating shocks decelerate moving weights smoothly, regardless of changes in energy-absorbing requirements, and they also have the tamperproof features of hydroshocks. In reality, self-compensating shocks exhibit minor variations in reaction force with variations in weights and velocities. These forces will be slightly higher than a properly tuned adjustable shock, but still well within acceptable limits.

One important difference, however, is that a single, self-compensating shock cannot cover the full effective weight range of an adjustable shock. For instance, an adjustable shock may handle a very large weight range of 10 to 10,000 lb, while a self-compensating model may be limited to weights of 200 to 1,000 lb. Most users find this range acceptable since the ratio of maximum to minimum weight is rarely more than 5 to 1.

Unlike hydroshocks, self-compensating units do not have a uniform orifice diameter. Rather, the size and location of each orifice is designed for a predetermined range of initial conditions. Such designs compensate for changing weight, velocity, temperature, and fluid compressibility.

The main advantage of self-compensating shocks is that they provide good deceleration even though input conditions change. This has a secondary benefit because input data does not need to be as accurate as with hydroshocks. The only disadvantage is the possibility of slightly higher reaction forces.

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