SCOPE

There are three sections in the Mechanical Power Press Standard where safety distances appear:  

1. section 1910.217 (C) (3) (iii) (e); 
2. section 1910.217 (C) (3) (vii) (c); and
3. section 1910.217 (C) (3) (viii) (c).

There are two basic formulas that cover three essentially different situations:

1. presence sensing on partial-revolution clutch machines;
2. two-hand control on partial-revolution clutch machines; and
3. two-hand trip on full-revolution clutch machines.

All formulas establish minimum safety distances from the safety device to the point of operation.  They are normally, but not exclusively, used in hand-feed situations where it is necessary to enter the point of operation.  The various terms used in the formulas are not identical, and the units or input data must be appropriate to the situation under study.

Section 1910.217 (C) (3) (iii) (e)

All formulas in this section applies to the required safety distance where presence-sensing devices are used.  It can only be used on partial-revolution clutch machines.  This formula gives the minimum distance from the sensing field to the point of operation.

Ds = (63 inches/second) x (Ts seconds)  

where Ds = minimum safety distance in inches

63 inches/second = hand speed constant

Ts = stopping time of the press, in seconds measured at approximately the 90 degree position of crankshaft rotation.  (At this moment the slide will be moving at its maximum speed.)

The formula is simple and only requires the substitution of a single value, “Ts”.  The increments of time dealt with on most mechanical power press applications are extremely small (probably milliseconds); they are not numbers that can be established by eye and the use of a stopwatch.  It will be necessary to use fairly sophisticated equipment to measure the stopping time.  The equipment can be either rented or purchased.  However, many manufacturing operations may have technical skills within their own operations that may permit them to devise their own equipment for this purpose.

Section 1910.217 (C) (3) (vii) (c)  

The formula in this section is identical to that in 1910.217 (C) (3) (iii) (e) except that it applies to the safety distance requirements between two-hand control devices and the point of operation.  It can only be used on partial revolution clutch machines.  This formula gives the minimum distance from each two-hand control device to the point of operation.  (Each multiple control station on any machine must comply.)  It should be noted that, “two-hand controls shall be fixed in position so that only a supervisor or someone from the safety department are capable of relocating the controls.”  Refer to OSHA section 1910.217 (E) (3) (vii) (d).

The difficulties associated with measuring the stopping time, “Ts”, are the same as previously mentioned.  The formula is:

Ds = (63 inches/second) x (Ts seconds)

Section 1910.217 (C) (3) (viii) (c)

The formula in this section applies only to full-revolution clutch machines.  It is similar to those previously discussed, at least in form.  There are, however, some important distinctions.  The various components of the formula have different subscripts (Dm) vs. Ds and Tm vs. Ts).  The Dm vs. Ds is of no significance; the difference between Ts vs. Tm, however, is important.

First, Tm is a number that can be calculated based on known crankshaft speeds, or it can be measured by use of a tachometer.  Secondly, its value will be affected by the number of engaging points in the clutch.

The following formal definition of Tm is from the Federal Register:

(Tm) is the maximum time the press takes for the die closure after it has been tripped (time is in seconds).  For full-revolution clutch presses with only one engaging point, Tm is equal to the time necessary for one and one-half revolutions of the crankshaft.

Tm = (1.5 revolutions) x (time in seconds of 1 revolution of crankshaft)

For full-revolution clutch presses with more than one engaging point, Tm should be calculated as follows:

Tm = (1/2 + __________1__________) x (time in seconds of 1 revolution of crankshaft)

                        No. of engaging points

Sample Calculations

The following assumed conditions will be used in the sample calculations:

• Crankshaft speed @ 60 RPM
• Stopping time for partial revolution clutch machine:  .080 seconds (in actual practice, the user must measure the “real time” required to stop the slide on each individual press.)

Engaging dogs for full-revolution clutch machines (example are shown for 1, 2 and 4 engaging dogs.)

Presence-Sensing and Two-Hand Controls

Refer to section 1910.217 (C) (3) (iii) (e) for presence sensing and section 1910.217 (C) (3) (vii) (c) for two-hand controls.

        Speed = 60 revolutions/minute divide 60 seconds/minute = 1 revolution/second

        Stopping time = Ts 

        Ts = .080 seconds for the brake to stop the slide

        Minimum Safety Distance = Ds

        Ds = (63 inches/second) x (Ts) = (63 inches/second) x (.080 seconds) = 5.040 inches

For the assumed condition used in this example, the safety distance for presence-sensing devices and two-hand control (as spelled out in the two sections involved) is 5.040 inches.

Full-Revolution Clutch Machines  

Refer to section 1910.217 (C) (3) (viii) (c) for full-revolution clutch machines.

Assumed speed = 60 revolutions/minute divided 60 seconds/minute

 = 1 revolution/second = 1 second/revolution

   A.  For one engaging point or dog @ 60 RPM:

      Stopping time = Ts

      Tm = (1.5 revolutions) x (time of 1 revolution of crankshaft)

       = (1.5 revolutions) x (1 second/revolution) = 1.5 seconds

      Minimum Safety Distance = Dm

      Dm = (63 inches/seconds) x (Tm) = (63 inches/seconds x (1.5 seconds) = 94.5 inches        (minimum safety distance for 60 RPM and one engaging point)

The calculated minimum safety distance arrived at in this example warrants comment because of the assumed values used.  In an actual situation, it might be an unrealistic solution to locate the two hand trip at this distance.  It would indicate that the methods as well as product and safety approaches be revised to arrive at a better solution.

   B.   For two engaging points or two dogs @ 60 RPM: