OSHA Machine Guarding: Safety Distance Formulas and Table O-10

Every guard and safety device on a mechanical power press has to sit far enough from the point of operation that a hand cannot reach the hazard before motion stops. The OSHA machine guarding distance chart and formulas at 29 CFR 1910.217 give you two tools for setting that distance: a timing-based formula for presence-sensing devices and two-hand controls, and Table O-10, which sets fixed opening-size limits for barrier guards.¹

The Safety Distance Formula for Presence-Sensing Devices and Two-Hand Controls

The regulatory formula is:

[latex]D_s = 63 \times T_s[/latex]

[latex]D_s[/latex] is the minimum safety distance, in inches, from the sensing field or the hand-control buttons to the nearest point-of-operation hazard.
63 is the hand speed constant in inches per second, the assumed maximum speed at which a hand can move toward the hazard after the safety device triggers.
[latex]T_s[/latex] is the press stopping time in seconds, measured at approximately the 90-degree position of crankshaft rotation.

Multiply how fast a hand can travel by how long the press takes to stop. If the press stops in 0.3 seconds, the sensing field must sit at least 18.9 inches from the hazard. The installed distance must meet or exceed that value.

The ANSI B11.1 Enhanced Formula

OSHA’s own eTool guidance recognizes that mechanical stopping time is only part of the total time between detection and a safe condition. The ANSI B11.1 press safety standard breaks that interval into its components and adds a penetration factor:²

[latex]D_s = K \times (T_s + T_c + T_r + T_{bm}) + D_{pf}[/latex]

K is the hand speed constant, still 63 inches per second.
[latex]T_s[/latex] is the stopping time of the press at the final control element.
[latex]T_c[/latex] is the response time of the control system.
[latex]T_r[/latex] is the response time of the presence-sensing device and its interface.
[latex]T_{bm}[/latex] is the additional time the brake monitor allows for normal variation in braking performance.
[latex]D_{pf}[/latex] is the depth penetration factor, the extra distance added because a hand or finger can partially pass through the sensing field before the device detects it.

The depth penetration factor depends on the light curtain’s minimum object sensitivity. A curtain that detects objects as small as 14 mm has a smaller penetration allowance than one that detects only objects of 30 mm or larger, because a finger can slip farther between widely spaced beams before triggering a stop.

The ANSI formula almost always produces a longer distance than the two-variable OSHA formula, because it captures control wiring lag, sensing-device lag, and brake-wear tolerance. Many employers use it as their working standard for that reason.

Two-Hand Trip Devices Use a Different Time Variable

Two-hand controls hold the same 63 × Ts formula as presence-sensing devices, because the operator must maintain pressure and the press can stop mid-stroke. Two-hand trip devices are different. Once tripped, a full-revolution clutch press continues through its cycle rather than stopping, so the timing variable is the closing time of the die, not the braking time:¹

[latex]D_m = 63 \times T_m[/latex]

[latex]T_m[/latex] is the maximum time the press takes to close the die after being tripped. For a full-revolution clutch press with a single engaging point, [latex]T_m[/latex] equals the time for one and a half revolutions of the crankshaft. For presses with multiple engaging points per revolution, the calculation adjusts downward based on the number of engaging points. The distance has to keep hands away for the whole closing cycle, not just a braking interval.

Table O-10: Guard Opening Size and Distance Chart

Fixed barrier guards and interlocked perimeter fences don’t rely on timing. They limit reach by controlling the size of any opening in the guard. Table O-10, at 29 CFR 1910.217(c)(2)(iv), sets the required distance from the point of operation for each opening size:¹

1/4 inch opening: 1/2 inch up to 1-1/2 inches from the hazard
3/8 inch opening: 1-1/2 to 2-1/2 inches
1/2 inch opening: 2-1/2 to 3-1/2 inches
5/8 inch opening: 3-1/2 to 5-1/2 inches
3/4 inch opening: 5-1/2 to 6-1/2 inches
7/8 inch opening: 6-1/2 to 7-1/2 inches
1-1/4 inch opening: 7-1/2 to 12-1/2 inches
1-1/2 inch opening: 12-1/2 to 15-1/2 inches
1-7/8 inch opening: 15-1/2 to 17-1/2 inches
2-1/8 inch opening: 17-1/2 to 31-1/2 inches

A tiny opening lets only a fingertip through, so the guard can sit close. A larger opening lets more of the hand or forearm reach in, so the guard has to sit much farther back. The jump from a 1-1/4 inch opening (minimum 7-1/2 inches away) to a 2-1/8 inch opening (minimum 17-1/2 inches away) shows how quickly required distance scales once forearm penetration becomes possible.

These limits apply to every opening on the guard, including slots, gaps between guard sections, and spaces around feed mechanisms. If any single opening exceeds the allowable width for its distance, the guard fails compliance.

Measuring Stopping Time Correctly

The formula is only as reliable as the stopping-time measurement you put into it. A stop-time measuring instrument captures the exact interval between the stop signal and the moment all hazardous motion ceases. OSHA expects only trained personnel to conduct this evaluation.³

Take the measurement multiple times and use the longest recorded stopping time in the formula. An average would understate the worst case. Measure at approximately the 90-degree crankshaft position, where the slide is moving fastest and the brake works hardest, and use the heaviest upper die to capture the most demanding condition.¹

This is where installations quietly fall out of compliance. Friction brakes wear, and stopping times creep up. A press that tested at 0.25 seconds during installation might be stopping at 0.32 seconds six months later. That difference pushes the required safety distance from 15.75 inches to 20.16 inches. If the light curtain has not moved, the setup is no longer safe.

Brake Monitors and Ongoing Compliance

Presses using presence-sensing devices or two-hand controls must have a brake monitor that watches braking performance on every stroke. If stopping time or braking distance degrades to the point where the installed safety distance no longer satisfies the formula, the brake monitor must automatically prevent another stroke.¹

The regulation limits the allowable drift. The brake monitor setting can permit no more than a 10 percent increase over the longest measured stopping time, or 10 milliseconds, whichever is longer.⁴ Once brake wear or other factors push stopping time past that limit, the press has to come out of service for adjustment, repair, or maintenance before it runs again.

Penalties for a Miscalculated or Drifted Distance

An improperly calculated or installed safety device is a citable violation. OSHA’s current maximum penalty for a serious violation is $16,550 per occurrence, and a willful or repeated violation can reach $165,514 per occurrence.⁵ These amounts are adjusted for inflation each January.⁶

A distance that was correctly calculated at installation but never rechecked after brake wear still counts as a violation if the current stopping time makes it inadequate. The same applies to a barrier guard with openings that once matched Table O-10 but has since shifted or been modified so the distance no longer holds. Inspectors measure what exists on the day of the inspection.

1
Occupational Safety and Health Administration. 29 CFR 1910.217 – Mechanical Power Presses
2
Occupational Safety and Health Administration. Machine Guarding eTool – Presses – Safety Distance
3
Occupational Safety and Health Administration. Utilization of a Stop Time Measuring Instrument for Verification of Compliance With 29 CFR 1910.217
4
eCFR. 29 CFR 1910.217 – Mechanical Power Presses
5
Occupational Safety and Health Administration. OSHA Penalties
6
Occupational Safety and Health Administration. Federal Civil Penalties Inflation Adjustment Act Annual Adjustments