Machine Design #28: Safe Machine Guard Design - Gaps, Sharp Edges and Pinch Points to Avoid
A machine guard is not just there "to look nice". A properly designed guard must prevent the operator from reaching the danger zone, must not itself create new risks of pinching, sharp edges or toppling when opened, and must still allow observation, cleaning and maintenance in a controlled way.
The most common mistake is to put a panel in front of a mechanism and assume the machine is now safe. If a gap still lets a finger through, if the door can open before the mechanism has stopped, or if the guard edge is sharp as a blade, that guard has not solved the real risk.
Start from the hazard, not from the guard shape
Before choosing sheet metal, polycarbonate or aluminum profile, identify who can access which area in every situation:
- normal operation;
- feeding blanks, taking products, clearing jams;
- daily cleaning;
- machine adjustment, die change, tool change;
- maintenance when air pressure, gravity, heat or residual electrical energy remains.
Then apply the risk-reduction hierarchy: eliminate the hazard by design if possible; if not, use a cover/guard; next comes an interlock or door lock; and only last, warnings and instructions. A warning label cannot replace a guard or an interlocking device.
1. Do not confuse the "anti-crush gap" with the "safety distance to the danger zone"
These are two different concepts.
The anti-crush gap applies to two surfaces moving relative to each other, where a hand or a body part could be pressed/pinched. ISO 13854:2017 gives minimum reference gaps by body part. When the risk is only pressing/pinching, the commonly used benchmarks are:
| Body part that can enter | Minimum reference gap |
|---|
| Finger | 25 mm |
| Hand/wrist | 100 mm |
| Arm | 120 mm |
| Leg | 180 mm |
| Head | 300 mm |
| Whole body | 500 mm |
The values above only handle the crushing/pinching risk. If there is a cutting blade, a cutting edge, a drawing-in motion, impact or flying objects, leaving only a "minimum gap" is not enough; you must use another measure such as full enclosure, increased distance, local shielding, an interlock or a safe stop.
The safety distance to the danger zone is the distance from a hole/mesh/slot on the guard to the dangerous mechanism inside. This distance depends on the size and shape of the opening, the direction of approach and the body part that can be inserted. ISO 13857:2019 has a separate table for this; you should not take one fixed number and apply it to every ventilation hole or every type of mesh.
2. Ventilation holes and observation windows: a small hole is not always safe
Ventilation holes, mesh, expanded metal and observation windows must all be checked against two conditions at the same time:
- Does the opening size let a finger, hand or arm pass through?
- If it can pass through, from the body's stopping point to the internal hazard, is there still enough distance?
For a local pinch zone on a conveyor or roller, a slot of no more than 6 mm is commonly used as a practical benchmark so a finger cannot enter. But the 6 mm benchmark cannot replace the entire ISO 13857 table for a guard with holes, mesh or a larger distance to the hazard.
When you must use mesh or expanded metal, note on the drawing at least: the material type, the actual opening size, the thickness/wire, the mounting direction and the distance from the guard face to the danger zone. Do not just write "protective mesh".
3. Fixed guards, hinged guards and locked guards: choose by access frequency
Fixed guard
Used for zones that need no access during normal operation. A fixed guard should only be removable with a tool. The purpose is not to make maintenance hard, but to prevent the operator from removing the guard by hand and then running the machine.
Check the following points:
- do not use a hand knob or quick-release latch for a danger zone that does not need to be opened frequently;
- a removable guard must not "hang loosely" after the bolts are removed, to avoid the false sense that it is safely mounted;
- large or heavy parts should be split up, given handles and a safe removal/installation method;
- account for vibration, impact force, flying objects and the possibility of someone leaning on it.
Hinged/interlocked guard
Used when frequent cleaning, product removal, adjustment or direct observation is needed. The door must have a defined closed state; when opened, the dangerous motion must stop or be prevented from starting per the safety design.
An interlock is not just fitting a door switch. You must evaluate the mechanism's stopping time: if a person can open the door before the blade, robot, rotating shaft or hot part has stopped in a safe state, you may need a door lock with a guard-locking mechanism, or another solution.
4. Sharp edges and pointed corners: the guard must not become a source of injury
The cut edge of sheet metal, the laser edge, a protruding screw head, a sharp bend corner and a punched hole can all cut a hand when the operator wipes the machine or does maintenance.
Design principles:
- Round the edges or break the burrs on every edge a person can touch.
- Do not leave a thread end, screw head or nut exposed on the operator's side if there is a reversed/safer mounting option.
- Do not create a "blade" with a sheet edge tapered to 0 mm; if a step change is needed, leave a flat face or use a bend corner.
- Handles, hinges, door latches and door edges must be checked in both the open and closed positions.
- A transparent guard that is cracked, aged or has a broken edge must also be treated as a mechanical risk.
5. Pinch-point locations that are often missed
It is not only gears and pulleys that pinch a hand. When reviewing a guard, also look for these zones:
- the door edge against the frame when the door closes;
- hinges, linkage bars, hand cranks and toggle mechanisms;
- rollers, belts, chains, sprockets and the transfer zones of a conveyor;
- lifting mechanisms, lead screws, rack-and-pinion;
- robots, rotary mechanisms and the zone between a moving part and the guard;
- guards lifted by a gas spring or heavy guards that can drop down;
- the gap between the guard and the machine frame when the guard vibrates or is struck.
An effective approach is to build the model at the travel-limit positions, not just the middle position. Pinch points often appear only when the mechanism reaches the end of travel or when the door is half open.
6. Choose the guard material by risk, not just by transparency
| Situation | Material/structure direction |
|---|
| Observation needed, no significant flying-object risk | Transparent panel with a suitable support frame; check aging, chemicals and heat. |
| Flying objects or impact possible | Assess impact resistance, frame connection and ejection direction; do not assume all plastic panels are equivalent. |
| No observation needed | Sheet metal or metal guard with suitable stiffness, connection and anti-vibration. |
| Ventilation needed | Punching, mesh or expanded metal, but check the opening size and distance to the hazard per ISO 13857. |
| Near a heat source | Check the surface temperature a person can touch and the continuous working temperature of the guard material. |
| Wash-down, chemical or food environment | Check chemical compatibility, cleanability, dirt-trapping gaps and the risk of cracking/staining. |
Do not choose a plastic panel just because it is transparent. The thickness, span size, frame, bolt positions, impact load and use environment together decide whether the guard is functional enough.
7. Guard design checklist before releasing the drawing
- Hazards: The pinch, drawing-in, cutting, impact, flying-object, heat and residual-energy zones have been identified.
- Access: Operation, cleaning, jam clearing and maintenance have been considered - not just the normal running state.
- Gaps: The anti-crush gap has been distinguished from the safety distance through a hole/mesh to the hazard.
- Holes and mesh: The opening size, distance to the hazard, material and mounting direction have been noted.
- Doors: Guards that need to open have an interlock/guard-locking suited to the stopping time and the residual risk.
- Sharp edges: Cut edges, sharp corners, screw heads and hand-contact positions have been handled.
- Structure: The guard has been checked at travel limits, vibration, impact, load and during removal/installation.
- Drawing: The opening direction, limit opening angle, handles, hinges, bolt datum and surface treatment requirements are shown.
- Confirmation: The safety decisions have been checked in the risk assessment for the specific machine.
Conclusion
A good machine guard does not only stop people from touching the mechanism. It gives the operator no reason to remove the guard to work, keeps maintenance controlled, and makes the residual risk clearly visible right from the drawing.
When designing, ask three questions: which body part can a person insert and where; what can they reach; and if the guard is opened or fails, what danger does the machine still have? Answering these three questions leads to a far safer guard than merely adding panels around the machine.
References
- ISO 12100:2010 - Safety of machinery, risk assessment and risk reduction.
- ISO 13854:2017 - Minimum gaps to avoid crushing of parts of the human body.
- ISO 13857:2019 - Safety distances to prevent hazard zones being reached by upper and lower limbs.
- ISO 14120:2015 - General requirements for the design and construction of fixed and movable guards.
- ISO 14119:2013 - Interlocking devices associated with guards.
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