Machine Design

Shocks bump up automation speeds

Today’s shock absorbers carry higher loads in smaller packages than previous ones, allowing their use in a wider range of applications.

Your shocks may be working fine now but when you notice leaky seals or noisy operation you could need new shocks. Although this may seem like a simple drop-in replacement, one should consider several options. Planned replacements, for example, are great opportunities to upgrade to new types of shocks that help machinery run better. This is also a good time to consider future plans. The machine in question may need bigger shocks or adjustable ones to accommodate growing output or faster speeds. Several manufacturers offer new equipment, software, and literature to ensure easy selection of the best shock for the job.

Shock-absorber basics
Since the advent of industrial shock absorbers in the early 1970s, manufacturers have increasingly depended on them for their numerous advantages. Shock absorbers lengthen machine life by reducing wear. Oddly enough, they increase operating speeds by quickly stopping moving objects. Shocks slow objects quietly and smoothly, which reduces noise and vibration and improves production quality. Their reliable operation helps machines run predictably, increasing safety.

Shock absorbers decelerate moving loads by converting kinetic energy to thermal energy. When loads hit shocks a piston rod slides into an oil-filled shock tube with orifices along its length. This forces fluid through the orifices, limiting motion and producing heat, which conducts out of the shock to the atmosphere. The total orifice area is the primary factor affecting shock response. That means larger or more orifices produce lower resistive forces than smaller or fewer ones.

The simplest shocks, called dashpots, have one orifice for fluid flow. These provide high initial damping forces that quickly diminish and, as such, are impractical for applications requiring smooth travel throughout the shock’s stroke. Multiple-orifice shocks have several orifices along the length of the shock tube. These produce smooth, even damping through the entire stroke. Nonadjustable models decelerate objects moving with the same force and speed every time they contact the shock.

When parameters vary, manually adjustable or self-compensating shocks are common choices. Self-compensating shocks have special orifice patterns on the shock tube that help compensate for varying loads. The shocks provide high initial damping for light, fast loads and high final damping for heavy, slow loads. Although high initial damping may sound similar to a dashpot, self-compensating shocks, unlike dashpots, provide smooth travel through the entire stroke.

To meet increasing demands for self-compensating shocks with high energy capacities, Ace Controls Inc., Farmington, Mich., recently introduced its SC2 Heavyweight Series shocks. Compared to traditional models, SC2 Heavyweights carry up to 950% more effective weight and have 280% higher energy capacities in the same package size. The soft-contact, self-compensating shocks are useful in applications that must decelerate heavy loads at low speeds. The design combines the piston and inner tube into one component acting as both the pressure creating and pressure-controlling device. “This increases effective piston diameters in the same size outer tube while retaining the performance advantage of sharp-edge orificing,” explains Mike Ferkany, engineering manager at Ace. “With this design, the piston-tube wall thickness is not nearly as critical as before.”

Self-compensating shocks, however, are not always the right choice. Some customers need to specifically change shock response depending on day-to-day job changes. When this is the case, adjustable shocks are the best choice. Although self-compensating shocks can decelerate a variety of loads, adjustable models often cover a broader range and can be set to match incoming loads.

Adjustable shocks have a dial mechanism that lets operators vary the orifice size to slow different objects. Manufacturers can recommend a setting based on load and speed. Even without this information, adjustments are straightforward. Enidine Inc., Orchard Park, N.Y., offers a simple method to properly adjust shocks. After installing the shock, adjust it to its lowest setting and cycle the shock. If damping is too soft the shock will not visibly decelerate and will bottom out with an audible “bang.” Gradually raise the damping force until the shock decelerates smoothly and makes little or no noise at the beginning and end of the stroke. Conversely, banging at initial contact indicates excessively high damping forces. When this happens gradually lower the setting until the shock operates smoothly and quietly. Switch to a larger model if the shock is at its highest setting and still decelerates abruptly at the end of its stroke. Try a smaller shock, on the other hand, when initial banging results with the shock at its lowest setting.

Demanding productivity
As demand increases for higher productivity, many industries need to move machines faster with smooth, accurate deceleration. To meet these demands, shock manufacturers have developed accurate, heavy-duty shocks for many rigorous applications. Robotic welders, for example, join metal parts for the automotive industry. Enertrols Inc., Westland, Mich., recently introduced a new line of high-precision metric shocks for automotive-welding applications. The heavy-duty shocks improve positioning accuracy from typical values of 0.005 to 0.01 in. to 0.002 in., according to Enertrols.

Heavy-duty operation is also a hot issue in bottle-making applications. The automated process sends molten glass into blowers that shape bottles for beverage industries. Until recently, however, extreme heat was beating up existing shocks. “Bottle-making companies were getting about two-months life out of previous shocks,” says one Enertrols official. “They switched to our Glass Shocks and have gotten three years of work from them.” The self-compensating shocks sustain extreme temperatures while maintaining accuracy.

Although all customers want rugged shocks, size is the critical factor for some. Industrial-robot manufacturers, for example, often need small, lightweight shocks with the same performance standards as larger models, according to Efdyn Inc., Tulsa. To do this Efdyn engineers reduced the number of parts while maintaining performance levels. “Pressure-vessel burst strengths must stay the same, so we can’t switch to aluminum or use thinner walls,” according to an Efdyn official. “We can’t use aluminum on any other shock parts either, because it won’t sustain reaction forces. So we have to eliminate parts to lighten our shocks.” Efdyn’s Minibuffer shocks address the demand for lightweight shocks. Available with fixed and adjustable cushioning, some models cycle up to 60 times/min. Others can sustain impact speeds up to 118 ips with 3,472 in.-lb absorption energy.

Shocks can be more than mechanical devices. For instance, sensors mounted on shocks detect piston-rod position and velocity. They can then adjust shock response to match impact forces. They can also send a signal to alert whether or not a shock is extended, or open other valves. Taylor Devices Inc., Tonawanda, N.Y., recently added electro-optical position sensors to its M-Series shocks. The specialty shocks mount to a satellite for end-of-travel deceleration of its rotating-optics system. Doug Taylor, president of Taylor Devices, has noticed an increase in customer requests for sensor-equipped shocks. “In 1967 we built our first shock with an electronic interface,” says Taylor. “We didn’t build another one until 1985. Now we build one or two a year. I wouldn’t call it a bona fide trend yet, but customers are certainly leaning toward a desire for electronic controls.”

Shocks shoot for smooth stops
Shock curves help describe shock-absorber operation. The ideal shock curve has nearly constant force throughout the shock’s stroke with rapid rise at the beginning and decline at the end of the stroke. Insufficient damping produces little force through the stroke and excessive amounts at the end (when the shock bottoms out). This comes from shocks with excessively large orifice areas. Rubber bumpers and springs produce similar damping.

Shocks with insufficient orifice areas produce high initial damping forces that diminish before the end of the stroke. This abruptly stops moving objects when they initially contact shocks. Dashpots, cylinder cushions, and valves work this way.

Self-compensating shocks adjust to force and speed inputs while maintaining acceptably smooth travel. The shocks can decelerate a range of loads and speeds. When low or fast-moving loads contact them, self-compensating shocks produce high initial forces that taper off near the end of the stroke. High, slow loads, on the other hand, cause the shocks to produce higher forces near the end of the stroke.

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