By Josh Cosford, Contributing Editor
Your first thought upon hearing the term shock absorber leads your mind to the cylindrical tubes mounted to the four corners of your vehicle’s suspension system. Other than the misnomer (“shocks”), your correct thinking applies to the term. Even the term “shock absorber” is a bit misleading, and industry professionals may prefer you to call them dampers. But just as another vehicle suspension component — the anti-roll bar – is sullied with “roll bar,” we’ve learned to live with such colloquial terms.
Vehicle shock absorbers don’t even absorb shock the way you assume. A vehicle with only four coil springs will absorb the impacts of bumps just fine — only once the spring compresses to absorb that bump, the energy stored in that compression releases to expand the spring past its previous resting point. If you’ve experienced a ride in a vehicle with “blown shocks” or driven in any American-made sedan from the 1980s, you’ll remember the wallowing that occurred after a large bump. The vehicle would continue to oscillate until the chassis settled down once again.
Springing into action
Springs naturally oscillate when attached to a mass spurred into motion. The oscillation rate depends on the weight of the mass and the spring’s constant or spring rate. The spring rate defines the compressibility of the spring, usually described in pounds per inch. For example, if 200 lb were loaded onto a spring, causing it to compress 2 in., the spring rate is 100 lb/in. A loaded spring will oscillate indefinitely in a vacuum with no friction, but on a vehicle, friction eventually tames the spring until it once again achieves balance.
Although spring motion is periodic, meaning it will continue to oscillate until prevented from doing so, you can view this motion as linear. Linear motion such as the forward travel of a vehicle will continue until acted upon by an outside force, assuming we ignore friction. For example, a vehicle uses brakes to convert its inertia into heat, slowing the vehicle to a stop. The same effect must be applied to a spring — its inertia must be converted to heat to slow the oscillation.
Most intuitively know that shock absorbers slow the spring’s oscillation (and therefore the vehicle’s bouncing) but are unaware they do so by converting that motion into heat. As a fluid power professional, you probably know better than most how fluid dynamics govern the behavior of shock absorbers and that friction from pads on steel is not the only way to convert motion into heat.
Restricting fluid motion under pressure offers a convenient and controllable energy conversion method. The kinetic energy comes from the imparted inertia created when a vehicle dives, pitches, rolls or hits a bump or hole. The pressure created inside the shock absorber must go somewhere, or the entire unit would just blow apart. Orifices inside the piston of monotube “shocks” restrict flow and convert the hydraulic energy into heat as the fluid passes under pressure.
A look at design
Shock absorbers are essentially small-bore hydraulic cylinders equipped (or drilled) with orifices in the piston. Those orifices’ combined cross-sectional area dictates the piston’s flow rate potential. The “compound orifice” of the drillings equates to an overall orifice size that adheres to any chart displaying pressure versus flow curves. The smaller the orifice size, the less flow across the shock absorber piston and the higher spring braking effect.
Larger orifices result in a more relaxed damping effect on the spring oscillation, while smaller orifices reign tighter control over the suspension movement. In real-world terms, you’d correctly expect a luxury vehicle to take advantage of the relaxed damping of larger orifice sizes. At the same time, sports cars wish their suspension movement highly damped to more quickly subdue the movement of their stiffer springs. High-performance shock absorbers use much more sophisticated technology than the simple example described above, ignoring modern examples such as magnetorheological dampers.
A primary high-performance shock absorber uses a monotube design alongside nitrogen charging, making it a sophisticated hydraulic component. Imagine placing a second floating piston in the space between the primary piston and the cap and then nitrogen charging the volume between the cap and floating piston, much like a piston accumulator. Add fixed flow control valves to each side of the piston, and you have a high-performance shock absorber.
The pistons are more complex than the machined round slug used in hydraulic cylinders, using multiple layers, plates and disc valves. Their combination allows different damping rates for compression and rebound while also allowing multi-stage damping. A low-speed orifice allows easy fluid flow for shallow dips or bumps, providing a gentle ride. When a harsh impact results in rapid movement, the disc valves close to direct fluid through the more-restrictive orifices, attenuating suspension movement more rapidly.
Twin-tube shock absorbers previously dominated the market, most notably because they became popular more quickly. Early suspension damping technology used two leather-covered sleeves mounted one inside the other, and clamping devices allowed mechanics to increase the friction as the assembly wore. Not a sophisticated component, modifications to its design led to the twin-tube shock absorber, where a valve assembly was added to the piston and the cap of the primary tube.
Twin-tube shocks were also popular because of their inexpensive cost to manufacture. Their barrel requires no precision machining, such as on the monotube design. Without a precision-machined tube, the floating piston designed to separate the nitrogen from the oil would have no chance of sealing. The twin tube design was also more prone to aeration as the nitrogen gas could quickly enter the primary tube, although some modern designs now employ a bladder.
Shock absorber applications aren’t limited to automotive applications, of course. Industrial shock absorbers find themselves on material handling applications, trollies, conveyor systems and even amusement rides — machines and applications where a moving object must slow quickly and safely without slamming or bouncing.
An ideal shock absorption damping rate is linear and should neither bounce off the rod end nor slam down against the shock absorbers bump stop. The deceleration rate should appear on a graph as a 45° downward slope with the initial velocity at the top and dead stop at the bottom. Without using the shock’s full stroke, you’re damping too quickly or not quickly enough. The shock absorber should come to a rest near (but not at) the bottom of the stroke.
Industrial shock absorber construction
The construction of industrial shock absorbers differs slightly from those used in automobiles. Both designs use orifices to meter the flow of oil, although where the oil is metered varies by design. The more popular multiple-orifice shock absorber uses an ingenious yet simple method to increase the damping response sequentially as the device nears its bottom position.
The shock tube equivalent to the pressure tube used in automotive looks similar until you discover the orifice drillings in the shock tube itself. Rather than equip disc valves and axially aligned drillings, the industrial shock absorber uses orifices along the tube length to increase damping force inversely proportional to the stroke length. For example, assume the tube has four drillings along its length, evenly spaced between the piston’s retracted position and the cap. Then, as the load compresses the shock absorber assembly and the piston travels down the length of the shock tube, fluid exits all four orifices under pressure, where a relatively light damping action takes place.
The piston covers the hole and continues past the first orifice, reducing the flow potential across the combined orifices. As a result, the shock absorber slows further with only three holes to pass pressurized fluid. The same effect occurs as the piston covers the second orifice, leaving only two left to evacuate fluid, once again reducing the velocity of the piston. Finally, when only the last orifice remains open, the load should have slowed proportionately relative to stroke length, and combined with the increased spring force, the load will stop.
Because a shock absorber is a differential cylinder, you fluid power professionals are intuitively wondering where the cap side fluid has gone since you know its larger volume cannot fill the less volume taking up space around the rod. As fluid exits the shock tube, it flows into a cavity between the tube and the primary cylinder. A hole in the shock tube ports to the rod’s area, so fluid entering there helps prevent cavitation. The rest is absorbed by a foam accumulator and stored until the piston retracts, drawing the fluid out and back into the shock tube.
Industrial shock absorbers come in downright tiny versions with perhaps less than 10-mm bore shock tubes, growing across various sizes and configurations up to 8 in. bore or larger. The smallest examples may absorb loads of 250 lb at a velocity of 50 in./sec or faster (although not simultaneously). Larger bore shock absorbers damp the movement of even the most enormous loads, quickly tackling 200,000 lb of force. Remember that the spring installed with the shock does the heavy lifting to handle a high load, and the shock absorber itself simply damps the motion of the spring.
I should mention that shock absorbers cannot simultaneously handle their maximum shock force rating with their maximum acceptable velocity. If you’re unsure, discuss your requirements with your shock absorber manufacturer of choice.
Ultimately, automotive and industrial shock absorbers are dynamic orifices designed to transform kinetic energy into heat. That damping may be against an oscillating load or simply one piston compression stroke. Either example offers a peek into yet another fluid dynamic principle that fascinates our industry.
Filed Under: Engineering Basics, Related Technologies