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Ergonomics of machine guarding (PDF 646 KB)

Introduction

Machinery safety remains a critical issue with many serious harm injuries each year occurring due to inadequate guarding of machines.

All machinery needs to be adequately guarded at all times and machinery and guarding must be maintained in a safe working condition.

Duty holders must take all practicable steps to ensure the safety of workers when using machinery.

The Health and Safety in Employment Act 1992 (the HSE Act) sets out the performance required by duty holders such as employers, employees, designers, manufacturers, suppliers and hirers of machinery.

The Australian Standard AS4024 (2006) Safety of Machinery represents current state of knowledge in relation to the safeguarding of equipment and should be referred to by duty holders as the primary standard against which to benchmark. Employers, suppliers, manufacturers and designers remain free to work to other standards but will need to demonstrate that they are capable of achieving an equivalent level of safety in the circumstances in which they are used.

Ergonomics and machine guarding

A critical aspect of machine guarding is to ensure workers cannot reach past the guarding into the machine. The way a worker uses and interacts with a machine (ergonomic principles) needs to be considered when deciding how to best guard a machine.

Typical ergonomic principles include:

• the nature of postures and movements
• the ease of physical operation
• the effects of noise or temperature
• the lighting environment
• the clarity and location of manual controls
• the design of dials, markings and displays.

All these aspects need to be considered along with the tasks being performed and the capacity of the people performing this work.

An important application of ergonomics is the use of data from studying human body measurements (anthropometric studies) in machinery design. This type of information helps when making decisions about the dimensions of access openings for:

• maintenance and repairs
• the space requirements for operators
• the safety distances and gaps to prevent contact of parts of the body with danger zones
• out of the ordinary events, such as a breakdown or product blockage.

Choosing guarding

Machine guarding can become useless if an operator can reach past it. The general way this occurs is when an operator can reach:

• upwards
• over a guard
• into the machine
• around or along a guard.

Reaching upwards

Any hazard assessment for machine guarding should take into account how workers could access the machinery. The operator may be able to raise themselves above floor level and access machinery that is normally out of reach. Workers could access the machine by standing on ladders, furniture or parts of machines. By doing this they may be able to reach into the dangerous areas.

Reaching over barriers

The height of a barrier or guard needs to limit an operator being able to reach over into the danger zone. Where the barrier is low, the worker can bend over and increase their reach longer than their arm.

Where the barrier is at armpit height, reach is equal to the length of the arm. If the barrier is above shoulder height, interruption is at the elbow, or when higher still, at the wrist or fingers.

Reasonable reach values – minimum recommended heights and distances for guarding

Table 1 shows the reasonable reach values (RRVs) that are the minimum recommended heights and distances for guarding. RRV calculate the minimum distance (C) for guarding in relation to the distance the dangerous part of the machine is from the floor level (A), and the height of the guard (B).

This is the minimum distance that guards of various heights should be from dangerous parts and is calculated to conform to the principles of average human reach.

When the guard is positioned close to the dangerous part, consideration must be given to the size of any opening in the guard.

When a dangerous part is between two heights in the left column of Table 1 (A), the height that provides the longer distance should be used when calculating distance from the guard. When a dangerous part is between two distances from the guard (C), the shorter distance should be used when calculating height of the guard (B).

The Australian Standard AS 4024 Safety of Machinery recommends longer distances in most circumstances and it is advisable to also refer to the relevant tables in this standard and apply the longer distances where practicable.

Reasonable reach values

Reasonable reach-over example

a. The height of the dangerous part from the floor is 610 mm.

b. A guard is 1220 mm high.

c. The distance the guard must be from the dangerous part is 530 mm.

Reach around barriers

When an operator’s arm reaches over or around a barrier, the action is described as a ‘reasonable reach curve’.

The ability to reach around barriers is determined by the distance of the elbow joint and wrist joint from the reach curve (e.g. from the finger tip). The average elbow joint is 460 mm from the finger tip. Therefore, if an additional barrier is placed with its edge only 430 mm (or less) from the curve, the forearm cannot be fully bent around it. This is illustrated in Figure 4.

When the edge of the barrier is 430 mm or less from the reach curve, the shaded area is “safe by position” since no part of the arm or hand can reach into it.

If the arm is pulled back until the wrist is near the edge of the additional barrier, the hand can be bent around the barrier and may reach the dangerous part if it is too close. Generally the hand will not reach as far as 200 mm around that point. The reach of a hand bent around the end of a barrier will therefore be a curve of radius 200 mm with a centre point at the end of the barrier.

With the barrier in each of these positions it is still possible for the hand to reach back underneath the barrier to a maximum of nearly 200 mm from the edge. The space below the barrier is unsafe, as well as any space in front of the barrier, which can be reached by the bent arm and hand.

When the edge of the additional barrier is only 200 mm from the reach curve, only the hand can be bent round it. Any dangerous part should be beyond a 200 mm radius from the edge of the barrier. If the barrier is less than 200 mm from the reach curve, the bending of the hand becomes more restricted, until at 80 mm only the fingers can be curled under the barrier.

If the fingers alone can be bent around the edge of the barrier, any dangerous part should be beyond an 80 mm radius from the edge of the barrier.

Openings, admitting the fingers or hand

If the opening will admit one, two or three fingers, the reach is restricted by the roots of the fingers (Figure 6). The distance between the guard and the dangerous part must be longer than the reasonable maximum length of the longest finger plus a clearance allowance.

If all four fingers can be admitted through the one opening, the size and shape of the opening will be an important factor in determining the extent of the reach. In some cases, reach will be limited by the root of the thumb, and in others by the thickness of the hand, the wrist or the arm. In a case where reach is restricted by the root of the thumb, the distance from dangerous parts necessary is the reasonable maximum length of a hand from fingertip to root of thumb, plus clearance.

The thumb can be folded within the palm of the hand, and the width of the hand in this case, is the width of the palm. The depth of the hand is increased by the thickness of the palm and thumb folded in. This limits the reach through potential depending on the width and depth of the opening.

The distance of the guard from the dangerous parts is determined by the maximum reasonable length of fingers and hand, or of fingers, hand and arm at different points.

If the opening can admit the whole arm and a portion of the shoulder (but excludes the head and trunk) the reasonable reach is assessed as the distance from fingertips to armpit and the minimum guarding distance should be not less than 840 mm.

Reaching safely through openings

There are many reasons to put an opening in a machine guard. The operator may need to insert or retrieve material, watch the work in progress, adjust the equipment or clean machinery.

Openings should meet the needs of production, but ensure that entry through the guard prevents reaching into danger. Hazards of the machine are isolated by restricting the distance a person can reach through the opening.

Safe separation distances for smaller openings are subject to rules depending on whether they are slots, squares, or circles.

Slots - elongated pararallel openings

There are five rules to determine the distance of the opening from the hazard, and the maximum permissible opening in the guard.

(1) Openings with a gap up to 6 mm can be treated as so small as to be insignificant. The guard is virtually the same as solid material and a working clearance of 25 mm is all that is required for safe    separation.

(2) Openings with a gap above 6 mm but no greater than 13 mm will admit part of a finger. The distance required is 50 mm which is part of a finger length plus a working clearance.

(3) Openings with a gap above 13 mm but no greater than 16 mm will admit a finger. The distance required is 100 mm which is a finger length plus a working clearance.

(4) Openings with a gap in between 16 mm and 82 mm are calculated from the formula:

    Where

“y”=x/10+6

 Where:

• y is the distance between sides of the slot, or opening size

• x is the safe distance from the outside of the guard to the hazard.

To find the safe distance x the formula can be written:

x=10(y-6)
 

(5) The distance between the guard and the dangerous part in the case of an opening between 82mm and 152mm should be at least 840mm, or arm’s length from armpit to fingertip plus a clearance.

Openings more than 152 mm are subject to the RRV over guards set out in Table 1.

Example of slot size and distance from dangerous part

If the distance of a guard from a dangerous part is 300 mm, find the maximum opening size of a slot.

x = 300 mm

y = X/10 + 6

y = 300/10 + 6

y = 36 mm

Square and round openings

Reach through square and round openings is restricted by the thumb. Restrictions on reach though square and round openings are explained in Table 3 below.

Square openings

(1) For square openings up to 38 mm, apply distances as given in Table 3. (These distances may also be applied to round openings of diameters equivalent to the squares indicated and to other shapes such as rectangles and diamonds with a diagonal measurement of up to 54 mm).

(2) Gaps in square openings from 38 mm up to 82 mm are subject to the formula:

• y is the distance between sides of the square, or opening size;
• x is the safe distance from the outside of the guard to the hazard.

To find the safe distance x the formula can be written:

 x=10(y-6)

(3) The distance between the guard and the dangerous part in the case of an opening between 82 mm and 152 mm square must not be less than 840mm.

(4) Openings in excess of 152 mm square are subject to the RRV over guards set out in Table 1.

Note: An opening 38 mm square is the maximum safe opening permissible at a distance of 100 mm from a dangerous part.

Example of distance from dangerous part for square

If the square opening is 80 mm find the required safe distance from the guarding to the dangerous part.

y = 80mm

x = 10(y - 6)

x = 10(80-6)

x = 10x74

x = 740mm

Round openings

(1) Openings up to 38 mm in diameter should use the table values in Table 3.

(2) For openings between 38 mm and 50 mm in diameter, the minimum acceptable distance between the opening and the dangerous part is 152  mm

(3) Openings from 50 mm up to 82 mm in diameter are subject to the formula:

Where:

“y”=x/10+6 

• y is the diameter of the opening;
• x is the safe distance from the outside of the guard to the hazard.

To find the safe distance x the formula can be written:

 x=10(y-6)

(4) For openings more than 82 mm in diameter, apply RRV over guards as set out in Table 1.

Where the opening is 50 mm square, a small hand can be inserted and the reach through values in Table 4 needs to be applied. The safe distance from a 50 mm square opening is 440 mm. In the case of an opening 45 mm square, a small hand can almost be squeezed through, but in any case the reach through this opening is at least 390 mm.

A small hand can be inserted through a 50 mm square opening. It is not possible to do this with a round opening of 50 mm in diameter where reach is restricted to 152 mm. The whole forearm can reach through if the diameter is increased to 55 mm.

Reach through openings

As an alternative to working through the rules of calculation above, you may prefer to simply use the lookup table below in Table 4. The table is a summary and a more precise measurement will require calculation.

References

Ministry of Business, Innovation and Employment (MBIE) Guidance

• Safe use of machinery - An introduction
• Safe use of machinery factsheets - Information, installation, operation and maintenance
• Safe use of machinery factsheets - General principles of machine guarding

Australian Standard (available from www.saiglobal.com)

• AS 4024: Safety of machinery (series)