Minimal Safe Distance Calculation

The Minimal Safe Distance Calculation shall guarantee that a person is not able to reach a dangerous machine part before the machine movement has stopped. This is calculated with the formula as called for from the ANSI B11.19-2003 Performance Criteria for the Design, Construction, Care and Operation of Safeguarding Standard.

K = The maximum speed at which an individual can approach the hazard, expressed in inches per second.

To quote ANSI B11.19-2003: “The factor K is the speed constant and includes hand and body movements of an individual approaching a hazard area. The following factors should be considered when determining K: a) Hand and arm movement; b) Twisting of the body or shoulder, or bending at the waist; c) Walking or running.

One of the accepted values for K is the hand speed constant (it is usually considered as the horizontal motion of the hand and arm while seated). Its common value is 63 in./s although other values (typically higher) are also used. The hand speed constant does not include other body movements, which can affect the actual approach speed. Consideration of the above factors should be included when determining the speed constant for a given application.”

Safety Light Curtains:

Dpf in mm = 3.4 x (Os – 6.875mm)

Dpf in inches = 3.4 x (Os – 0.275”)

Where:

Os = minimum object sensitivity or resolution.

For horizontal mounted Safety Light Curtains:

Dpf in mm = 1200mm

Dpf in inches = 48”

For multi beam Safety Light Grids:

Dpf in mm = 1200mm for a 2 beam system with beams spaced 600mm apart.

Dpf in inches = 48” for a 2 beam system with beams spaced 600mm apart.

Dpf in mm = 900mm for a 3 beam system with beams spaced 450mm apart.

Dpf in inches = 36” for a 3 beam system with beams spaced 450mm apart.

Example 1: With the aid of the Jokab Smart Stop Time Analyzer, a mechanical power press has a measured stopping time (Ts + Tc) of 325 ms. This includes both the stopping time of the machine as well as the stopping time of the control circuit. The response time of the Focus Safety Light Curtain F4-35-900 with 35mm (1.38”) resolution and 900mm (35.43”) protective height is 20ms (Tr). The stop time break monitor is set for 400 ms. In this example the safety light curtain must be mounted no closer than 767.63mm (30.22”) from the hazardous pinch point.

Tspm = 400ms – (Ts + Tc) Tspm = 400ms – 325ms Tspm = 75ms Dpf = 3.4 x (35mm – 6.875mm) Dpf = 3.4 x (1.38” – 0.275”) Dpf = 3.4 x 28.125mm Dpf = 3.4 x 1.105” Dpf = 95.63mm Dpf = 3.76“

Ds = [K x (Ts + Tc + Tr + Tspm)] + Dpf Ds = [1.6m/s x (325ms + 20ms + 75ms)] + 95.63mm Ds = [63inch/s x (325ms + 20ms + 75ms)] + 3.76” Ds = [1600mm/s x 420ms] + 95.63mm Ds = [63inch/s x 420ms] + 3.76” Ds = [1600mm/s x .420s] + 95.63 Ds = [63inch/s x .420s] + 3.76” Ds = 672mm + 95.63mm Ds = 26.46” + 3.76” Ds = 767.63mm Ds = 30.22”

Example 2: With the aid of the Jokab Smart Stop Time Analyzer, a robotic loader has a measured stopping time (Ts + Tc) of 175 ms. This includes both the stopping time of the machine as well as the stopping time of the control circuit. The response time of the horizontally mounted Focus Safety Light Curtain with 35mm (1.38”) resolution is 24ms. The depth of penetration factor is fixed at 1200mm (48”).

In this example the horizontal safety light curtain must be at least 1518.4mm (60.54”) from the hazardous pinch point.

Ds = [K x (Ts + Tc + Tr)] + Dpf Ds = [1.6m/s x (175ms + 24ms)] + 1200mm Ds = [63inch/s x (175ms + 24ms)] + 48” Ds = [1600mm/s x 199ms] + 1200mm Ds = [63inch/s x 199ms] + 48” Ds = [1600mm/s x .199s] + 1200mm Ds = [63inch/s x .199s] + 48” Ds = 318.4mm + 1200mm Ds = 12.54” + 48” Ds = 1518.4mm Ds = 60.54”

Example 3: With the aid of the Jokab Smart Stop Time Analyzer, a transfer gantry system has a measured stopping time (Ts + Tc) of 212 ms. This includes both the stopping time of the machine as well as the stopping time of the control circuit. The response time of the Focus Safety Light Grid F4-K3-900 with 300mm (11.81”) beam spacing and 900mm (35.43”) protective height is 14ms (Tr). For a 3 beam Safety Light Grid the depth of penetration factor is fixed at 900mm (35.43”).

In this example the safety light curtain must be mounted no closer than 767.63mm (30.22”) from the hazardous pinch point.

Ds = [K x (Ts + Tc + Tr)] + Dpf Ds = [1.6m/s x (212ms + 14ms)] + 900mm Ds = [63inch/s x (212ms + 14ms)] + 35.43” Ds = [1600mm/s x 226ms] + 900mm Ds = [63inch/s x 226ms] + 35.43” Ds = [1600mm/s x .226s] + 900mm Ds = [63inch/s x .226s] + 35.43” Ds = 361.6mm + 900mm Ds = 14.24” + 35.43” Ds = 1261.6mm Ds = 49.67”