Machine Safeguarding Technology: Keep Operators Out of a Pinch

By Carrie Halle, Contributor

Machine SafeguardingPinching your finger in a door can be painful but certainly not life-threatening. Pinch point injuries involving industrial machinery are another story—one that rarely has a happy ending.

A pinch point is created when two objects come together, and there is a possibility that a person could be caught or injured when coming in contact with that area. If any part of the worker’s body (typically hands or fingers) occupies that space during the pinching movement, there is a high probability of injuries, such as fractures, amputations, contusions, lacerations or even death. Machinery parts move at high rates of speed, making it difficult for an individual to pull free, once caught.

Pinch point injuries often occur as the result of workers who are not properly trained or don’t realize the dangers of machinery; or who take shortcuts to get the work done more quickly but end up injuring themselves, instead. Other common causes are:

  • Loose clothing, hair or jewelry becoming entangled in rotating parts, thus drawing fingers, feet and hands into a pinch point
  • Poor condition of equipment and lack of machine safeguarding
  • Not wearing proper PPE
  • Dropping or carelessly handling materials or suspended loads
  • Placing hands where they can not be seen
  • Not using the proper work procedures, include LOTO
  • Reaching into moving equipment and machinery to clear debris

A pinch point injury can be the result of something as large as a hydraulic press or as small as a pair of wire strippers. Of the thousands of disabling accidents that happen on the job each year, one third of them are hand injuries; of those injuries, approximately 80% are caused by pinch points.

In any plant or machine shop, there are machines and activities that can cause workers to be seriously injured by pinch points. Many of these hazards are avoidable, but some are not. For these unavoidable hazards, it’s critical to protect workers with machine safeguarding, as well as to keep them aware and alert of the hazards by providing training and posting safety labels.

Riveters & Welders

For the purpose of this article, we will be looking at “Good, Better, Best” approaches to safeguarding two of those types of machines that present unavoidable pinch point hazards: riveters and welders.

It is important to appreciate the magnitude of force between electrodes or tips. Depending on the material thicknesses being spot-welded, a strong weld joint requires a forging pressure ranging from several hundred to several thousand pounds. For example, a welding machine with a ¼-in electrode face set with 600lbs of electrode force will develop 12,230lbs/in2. This can cause major crushing damage to the operator’s finger, if caught at this pinch point.

We will not be examining light curtains, as they are rarely practical with hand-fed welding or riveting, since the user’s hands will normally be in the point of operation. Also, light curtains cannot usually handle parts that have flanges that are in the same area as the operator’s hands.

Good: Drop-Probe Devices

Drop-probes provide economical, simple-to-understand/use, reliable protection for the operator. Drop-probe devices function by allowing a sensing probe to drop by gravity around the point-of-operation hazard of a riveter or welder, prior to each intended machine cycle. If the sensing probe encounters the operator’s fingers and fails to drop to a preset position, the machine cycle cannot be initiated until the obstruction is cleared and the cycle re-initiated. However, if there are no obstructions to prevent the sensing probe from dropping, then the control unit will allow the machine to cycle when the probe reaches the preset position.

Users can fabricate a custom “drop ring” to fit narrowly around the point of operation, thus allowing the operator to hold the workpiece close for joining. Additional material, such as acrylic or polycarbonate clear plastic pieces or tube-type materials, can be added to the probe to provide adequate protection.

On machines that are mechanically operated, an air cylinder or electric solenoid may have to be added to the operating linkage to trip the machine. On machines that are hydraulically or pneumatically operated, the drop-probe control can usually be interfaced with the existing control system.

One drawback of drop-probe devices is they can only keep a machine from starting and do not stop a machine that is already in operation. For this reason, they must only be used on machines such as small riveters that perform a single, rapid movement each time they’re activated. Also, this type of system is not usable where the parts being welded are not flat in the area of the weld.

Better: Adjustable Stroke Drop-Probes

Identical in most ways to standard drop-probes, adjustable models offer an externally adjustable stroke, via a clamp collar, typically from 0-4.0 inches to accommodate any fixture, tooling or changes in the profile of the workpieces.

Best: Continuity Monitoring 

Available from only one manufacturer, Unitrol, this unique and highly effective system measures electrical continuity between the two electrodes to verify they are actually touching the part to be welded. If anything, such as the operator’s finger, blocks the movement of the electrode, the system will not detect continuity. In other words, it must detect metal between the copper tips rather than a finger. The continuity signal is picked up from points on the welding transformer secondary pads, eliminating the need for wire connections at the electrodes.

In operation, the electrodes are closed under low force, a process that is accomplished by a modified pneumatic valve system. A group of precision pressure regulators in a pad-locked box provides pressure needed to counterbalance heavy welding rams to less than 50lbs of force at the electrodes. If continuity is detected before a maximum time has been reached, the electrodes are brought to full welding force. However, if continuity is not detected before that time, the electrodes are retracted and will not come forward until the initiation switch has been released and closed again.

Other practical steps to safeguard operations are:

  • Install a pinch point warning sign on the machine
  • Train operators properly regarding the importance of keeping their hands away from movement
  • Make sure the foot switch used for machine initiation has a top and side shroud installed to prevent accidental activation
  • Require everyone in the welding department to wear safety glasses with side shields at all times
  • If possible, have the operator hold the parts being welded with both hands while the assembly is manipulated in the throat of the machine to access all weld locations

Since pinch points are the root of such a large number of hand injuries, it’s best to install viable pinch point safety on two machines where these injuries commonly occur: resistance welders and riveters. WMHS

Carrie Halle is Vice President of Marketing and Business Development at Rockford Systems, LLC. (www.rockfordsystems.com)