Licensed Professional Engineers
FORENSIC CLUES # 51 - "Nip Points" by John L. Ryan
A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 51: Vol. 1 October/November 2012
© 2012 M.A.S.E. LLC
( 479) 549-4860
A nip point is a hazardous area created by two or more mechanical parts rotating in the same plane in close proximity to each part, in opposite directions. These are also referred to as in-running nip points. The hazard here is the potential to get pulled into the machinery. Some nip points are designed to pull material into them in order for a machine to perform some alteration on the material, while some nip points are the result of power transmission and are not specifically designed to pull material into the machine. When an object, piece of clothing, or part of a person’s body contacts the nip point, the drive components will pull the object into the machine, when the forces acting on the object exceed the forces preventing the object from moving. The drive components will place resultant forces on the object perpendicular from the rotational point of the drive. For an object to get pulled in, there must be some sort of interference between the object and the drive components. The turning drive components will exert a frictional force on the object, the magnitude dependent on the interference and the amount of force being transmitted based on the this interference. The fixed drive components will pull the object into the machine when there is enough rotational friction force to pull the object or person into the machine.
In-Running Nip Point
Catastrophic injuries often result from being pulled into an in-running nip point. Depending on the distance between the drive components, the nature of the drive components’ surface characteristics, and the amount of force driving the machine, a person’s entire body can get pulled into a machine, or it may be limited to a limb. When a limb is pulled into a nip point, the body of the person may act as a physical barrier that prevents the rest of the body from becoming pulled in. Degloving injuries can occur after the person’s limb is pulled into the nip point. When the body or limb of the victim stops further entanglement in the hazard, the rotating drive mechanisms may continue to cause further injury and damage.
Gears Creating Nip Point
Examples of nip point hazards can be found on the drive mechanisms of conveyors, gears and pulleys. These mechanisms are extremely dangerous as a machine operator can get pulled into the machine, causing potential catastrophic body damage. Machine operators have been pulled into nip points when loose clothing became entangled in the nip point. Often there are large amounts of force driving the components in nip points, as is the case in the following figure. Any accidental contact with a high power nip point will result in extreme physical mutilation or death.
Unguarded Nip Points
Nip points can also involve relatively lower power components. While these nip points may not be life-threatening it is still imperative to provide adequate safeguards. Even office equipment like copy machines have low power nip points that can still be hazardous, and result in finger injury.
Nip points do not always involve two rotating elements. Nip points also occur where there is one rotating element, and a stationary object in close proximity to the rotating element. The mechanism is the same although the chances of becoming caught in the nip point is reduced – due to the fact that an object must get wedged into the actual nip point, which is a set size. In-running nip points with two rollers are much more hazardous because there is a large range of sizes of objects that can become caught in the nip point. Even so, nip points with one rotating element can still cause serious injury or death. Examples of this kind of nip point are found on grinding machines, drill presses, and other machinery with a single rotating element.
Nip points can also occur where there is a rotating element and a linearly displaced material. Conveyors have nip points where the drive roller meets the conveyor belt – anything caught in this point will become sucked into the nip point. This point on conveyors is especially hazardous as conveyor motors are often high-powered. Fatal accidents occur when someone is pulled into these high power nip points.
Grinding Disc Machine with Nip Point
What About Standards?
There are individual standards for each type of machinery. Normally these standards require a high degree of safeguarding, and do not allow unguarded nip points. Some manufacturers simply fail to properly safeguard their machinery. Older machines may not have the same standard of safeguarding, although often adequate technology was available to effectively safeguard the hazards. Some safety features may have been made optional by the manufacturer, and the customer chose to not have the safeguards. The initial sate of used machinery can be challenging to determine. Many accidents occur during maintenance procedures, where special precautions and safeguards must be in place. Maintenance procedures often bring personnel in close proximity to hazards. Sometimes machine safeguards will be bypassed to make maintenance procedures quicker, or to increase production speed.
Machinery that is not properly guarded is unacceptable, and violates industry standards and engineering design principles. Call or email us today to help you with your machine guarding cases.
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© 2012 Mechanical and Safety Engineering
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