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Real Performance Machinery Screen Printing Article :

Proper Shock Treatment Extends Part Life

Proper Shock Treatment Extends Part Life
By Rick Fuqua, Contributing Editor

Understanding what shock absorbers do and knowing signs of problems safeguards pneumatic press parts.Shock—or impact at energy levels high enough to cause damage—is a prime cause of fatigue and premature structural failure in air-operated (pneumatic) screen printing machines. The ability to slow and stop components, such as the print carriage, that move at high speeds is critical.

On air-operated machines, the squeegee/floodbar carriage or indexer table is pushed back and forth by an air cylinder, with the length of the cylinder stroke dictating the amount of travel. When the cylinder is fully extended, it reverses direction to retract the device. (This is notably different from a camdrive unit, which rotates 360° in the same direction without reversing.) The parts it moves change directions by following the cam and typically are pushed out for 180° of the rotation and pulled back for the complement, allowing speed without impact.

A linear decelerator prevents impact or shock at the end of a stroke by absorbing energy to decelerate motion in a linear fashion. The linear decelerator is essentially a piston partially protruding from a housing it is forced into by a part it impacts. The decelerator's resistance to being pushed into the housing slows the part's movement. The piston must collapse quickly enough to prevent impact, but slowly enough to provide linear resistance.

A shock absorber provides for a gradual letdown or linear reduction of speed by moving oil through holes inside the body of the housing as the piston is pushed in.

For the first half of the piston's travel, oil is free to escape. But as the piston collapses, it progressively covers these pathways so the oil has fewer and fewer escape passages. The further the piston collapses, the greater its resistance to movement. While this resistance is not linear, the way it slows the part it contacts is.

Adjustable linear decelerators allow the openings of these internal holes to be increased or decreased to enable the same shock to deal with a greater range of forces. The adjustment of a shock is critical, and care should be taken when making external adjustments on shocks equipped for this.

The shock setting should be reduced if the impact against a part appears to be too hard. The reverse is true if the shock appears too soft, quickly collapsing when the part comes in contact with it. The correct soft/hard setting rapidly and uniformly slows the part that comes in contact with the shock, with no impact at the beginning or end of travel and a seemingly linear motion.

Things that affect a properly adjusted shock include:

  • Substantial changes in the weight of moving parts (e.g., a switch from children's to larger or oversized platens or the installation of wider squeegee/ floodbars on the print carriage);
  • Changes in air pressure that increase the speed at which parts move, creating higher energy levels; and
  • Environmental changes, such as temperature or time that can cause changes in variables such as the oil's viscosity. (For some applications, shock manufacturers recommend reservoirs that give the rotating portion of the oil time to stabilize for more uniform performance.)

Recommendations for boosting the longevity and performance of shocks include ensuring the shock piston contacts the part at an angle no greater than 5°. (Since the piston is extended at the time and point of contact, hitting the part at a greater angle puts side loads on the piston that could prematurely wear the seals, resulting in leaks.)

On some machines, the print-head shocks are mounted to movable front-and rear-stroke adjustment plates. These plates or brackets sometimes have some slop in their fit to make them easy to move. If this slop is not taken into account, the plate can become cocked, in effect, placing the shock at an angle to the print carriage. If great enough, this angle can cause premature wearing of the shock. This can be prevented by using marks or measurements on the head to ensure these stops are not placed at an angle during reinstallation.

Another important point is that most shocks are not designed to be "bottomed out." That is, the piston should not be completely collapsed into its housing by the device pushing on it. In most cases, the piston shock should remain extended 1/16 inch. This ensures against internal shock and seal damage.

Shock manufacturers typically assume product users will provide positive stops so the moving part does not rest on the shock. Some manufacturers offer a collar that screws onto the outer body of the shock to keep the piston out 1/16 inch, but others have ignored the problem. Depending on the shock type and branch, shocks that bottom out may work for a while, but have a shorter life expectancy.

A shock may be suspect if it:

  • Leaks oil,
  • Collapses too easily and can't be adjusted to a harder setting, or
  • Can't be adjusted for smooth deceleration.

The key to replacing a shock is putting the new one in exactly the same place. Typically, the outer body of a shock absorber is threaded and mounted into a compatible receiving hole on the press. It is rotated to be set in or out; how far in or out it is determined by where the contacting part stops. In the most popular air-operated machines, for example, the main-drive shock determines the stopping place of the indexer platens. When the shock is fully compressed and the indexer is in contact with it and resting on its stop collar, the indexer must be aligned with the registration forks. This is because the machine goes up into the registration forks when it is fully extended by the drive unit. Failure to align these components can jam the press and damage or bend its forks, bearings, or arms.

Also, if the shock is adjustable, the settings should be fine-tuned before the first index. If the settings are off too far, even one bad index can cause serious damage. Therefore, it's better to replace the shock with the same type unit and duplicate the settings of the old shock when it was working properly.

Other failures can result from air or contaminants in the shock that cause it to perform improperly. Air in the oil causes problems because the air is compressible and the oil is not. Random air bubbles can cause inconsistent, soft results, although such problems typically don't arise other than with new shocks or when replacing oil in old ones.

Contaminants such as dirt can clog the oil holes, causing performance problems such as the shock being too "hard." This, too, is more likely to occur with a new or refurbished unit. In either case, repairs should be made according to manufacturer recommendations to avoid further problems.

If you suspect a problem with a shock, you should address it immediately, because excessive impact can damage parts very quickly.

Most air-operated machines use shocks in two areas: the front-and-back head drive and the main indexer drive's forward or extended stroke. Head shocks can be tested by exhausting the air off to the head, then manually moving the carriage until it impacts the front or rear shock. Push the carriage slowly to start. If the shock is working, build up speed to test it under a higher impact or load. The shock piston should collapse slowly. If it provides little or no resistance, it is suspect.

Note, however, that resistance should not be great if the impact is low, and the piston should self-compensate by being harder to push in at higher impact levels. (To get a feel for this, push in the head shock piston with your finger. You should feel progressively more resistance the faster you push.)

Head shocks normally are not designed to be repaired. Shock failure on a head can put that head out of commission, but the others can be run without it. This is not true of the main drive shock.

The main drive shock for the indexer tables is set to absorb a much higher energy level; as a result, it is difficult to test other than by watching it cycle. Because this part is essential to your pneumatic press and its life is finite, you always should have a new one on hand. You can test an old shock by substituting it and comparing performance. When your shock is new and working properly, be sure to note the hard/soft adjustment with various platen sizes and weights so no matter what platens are up when a shock fails, the settings on the replacement will be close enough to avoid parts damage.

Most of the large shocks in the drive area can be repaired. Parts and procedures for repairs can be obtained from the manufacturer, but the low cost of these shocks may make replacement preferable.

Real Performance Machinery L.L.C.

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