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Machine Guarding
Presses

29 CFR 1910.217(c)(3)(iii)(e):  the SAFETY DISTANCE (Ds) from the sensing field to the point of operation shall be greater than the distance determined by the following formula:

Safety Distance (Ds)
  Ds = 63 inches/second  x   Ts where:

  Ds = minimum safety distance (inches);

  63 inches/second = hand speed constant; and

  Ts = stopping time of the press measured at approximately 90°
  position of crankshaft rotation (seconds).

29 CFR 1910.217(c)(3)(viii)(c): The safety distance (Dm) between the two-hand trip and the point of operation shall be greater than the distance determined by the following formula:

Safety Distance(Dm)
Dm = 63 inches / second x Tm where:
Dm = minimum safety distance (inches):
63 inches / second = hand speed constant; and

Tm = die closure after it has been tripped (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. For full revolution clutch presses with more than one engaging point, Tm shall be calculated as follows:

    [   1 ] time necessary to complete one revolution of the crankshaft (seconds)
Tm = 1/2 +
  Number of engaging points per revolution  

Safety Distance Chart
T Ds
Ts Ds
.055   --   3"    .317 --   20"
.063   --   4"   .325 --   20"
.072   --   4"    .333 --   21"
.079   --   5'   .341 --   21"
.087   --   5"   .349 --   22"
.095   --   6"   .357 --   22"
.103   --   6"   .365 --   23"
.111   --   7"   .373 --   23"
.119   --   7"   .380 --   24"
.126   --   8"   .388 --   24"
.134   --   8"   .396 --   25"
.142   --   9"   .404 --   25"
.150   --   9"   .412 --   26"
.158   --   10"   .420 --   26"
.166   --   10"   .428 --   27"
.174   --   11"   .436 --   27"
.182   --   11"   .444 --   28"
.190   --   12"   .452 --   28"
.198   --   12"   .460 --   29"
.206   --   13"   .468 --   29"
.214   --   13"   .476 --   30"
.222   --   14"   .484 --   30"
.230   --   14"   .492 --   31"
.238   --   15"   .500 --   31"
.246   --   15"   .507 --   32"
.253   --   16"   .515 --   32"
.261   --   16"   .523 --   33"
.269   --   17"   .531 --   33"
.277   --   17"   .539 --   34"
.285   --   18"   .547 --   34"
.293   --   18"   .555 --   35"
.301   --   19"   .563 --   35"
.309   --   19"   .571 --   36"

The minimum safety distance is defined as the minimum distance from the light curtain's plane of light to the closest hazard or danger point where the operator could reach into the hazard.

This minimum safety distance is based on the stopping ability of the machine and a hand speed constant. When the minimum safety distance is calculated, several other factors must be taken into account, which are not included in the in the OSHA formula. These factors include the total system response time, the minimum object sensitivity of the presence sensing device, and the hand or object speed. The total system response time includes the stopping time of the machine under worse case conditions, response time of the control system, response time of the presence sensing device as stated by the manufacturer,  the response time of the interface, and, if applicable, additional time allowed for the brake monitor to compensate for variations in normal stopping time. Another factor includes the penetration distance (Dpf), which is based on the light curtain's MOS (minimum object sensitivity). The following formula is used to compute the minimum safety distance (Ds) on mechanical power presses to meet the ANSI (American National Standards Institute) B11.1 Press Safety Standard:

ANSI Minimum Safety Distance (Ds)

Ds = K x (Ts + Tc +  T+  Tbm) + Dpf where: K  = Hand speed constant (63 inches/second) Ts = Stop time of equipment measured at the final control element Tc = Response time of the control system T= Response time of the presence sensing device and its interface Tbm = Additional time allowed for the brake monitor to compensate for variations in
           normal stopping time

Dpf  = The added distance due to the penetration depth factor (MOS). Note: If the
           channel blanking feature is used on light curtains, additional safety distance
           must be enforced based on the number of channels blanked.

When determining the safety distance, a portable or built-in stop-time measuring unit must be used to check the stopping time (Ts) of the machine.

Stop-Time Measurement Device

When determining the safety distance, a portable or built-in stop-time measuring unit must be used to check the stopping time (Ts) of the machine. The STM (stop-time measurement) device measures the time it takes a machine to stop after a signal is given. It is mainly used on reciprocating (stroking or cycling) machines, such as mechanical and hydraulic presses or press brakes. With optional accessories, it can also be used on machines that rotate, such as lathes, mills, and drills.

Industry uses this type of device to find the stopping time of a machine before installing safeguarding devices such as a two-hand control or a presence sensing device. The stopping time measured by the STM device during the hazardous portion of the cycle is used in the OSHA or ANSI formulas to calculate the safety distance. The safety distance is then used to establish the location of the safeguarding device in relation to the nearest hazard. This device can also be used to periodically check the machine's stopping time to ensure that the current safety distance corresponds to the current condition of the machine's stopping ability.

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