A ‘machine guarding’ means any enclosure, barrier, or device constructed to prevent a person or his clothing from coming in contact with dangerous parts of the machine. The ‘point of operation’ is that part of the working machine at which cutting, shaping, forming, or any other necessary operation is accomplished. A guard for it is known as the point of operation guard.
One of the major goals of ‘Safety Engineering’ is the guarding of all machinery and equipment to eliminate personal hazards created by points of operation, ingoing nip points, rotating parts, flying chips, and sparks. Thus machine guarding is one of the basic steps to eliminate hazards. The machine should be so designed that all safety points are incorporated by the built-in safety principle.
The guards remove workers’ fear and thereby increase their morale and production. Guards are essential as an engineering and built-in control to prevent accidents when other precautions fail. The absence or defect of the guard is the main cause of accidents due to moving machines or their dangerous parts.
Table of Contents
Types of machine guarding
- Fixed guard
- Interlocking guard
- Adjustable guard
- Self-adjusting guards
- A fixed guard is the permanent part of the machine.
- Guard opening and its distance from the dangerous part should be fully safe.
- They should be a close fit, robust, and withstand speed, vibration, impact, etc.
- They should be properly fitted with clamps, bolts, etc.
- They require a special tool for their removal.
- Not dependent on any other part to perform the function.
- Usually made of sheet metal, screen, bars, or other material which will withstand the anticipated impact.
- Simple and durable.
Fixed guards are of four types:
- Fixed Total enclosure.
- Fixed total enclosures are used to cover prime mover and transmission parts such as headstock gearing, belts, pulleys, etc.
- Fixed limited access guard.
- Fixed limited access guard provides a minimum opening in the guard to insert and remove the job (material). It disallows the finger to trap. If the material is jammed, it requires a special tool for opening and refitting the guard. They are used on power presses.
- Fixed adjustable access guard.
- Fixed adjustable access guard provides opening through or around the guard to accommodate materials of different sizes. Guards on band saws, jigsaws can be raised or lowered as per the thickness to cut. Such guard on a circular saw or drill tool adjusts automatically as the job or tool moves. Its disadvantage is, it gives little protection for thin material.
- Distance or barrier guard.
- Distance or barrier guards make it physically difficult to gain access. Fixed railing to planning/shaping machines is this type.
Interlock guards make certain that the machine cannot be operated until the guard screen is in position and conversely the guard cannot be removed until the working parts have stopped.
- Usually connected to a mechanism that will cut off the power automatically.
- Could use electrical, mechanical, or hydraulic systems.
- Should rely on a manual reset system.
There are four types of interlocking guards:
- Mechanical interlocking.
- Electrical interlocking.
- Trip guard.
- Control guard.
- Mechanical and Electrical interlocking guard:
- mechanical, electrical, hydraulic, or pneumatic systems are used to actuate the guard. The mechanical interlocking of a card machine and electrical (limit or micro switches) interlocking of headstock covers of many textile machinery are such examples.
- Trip guard:
- Trip guards are actuated by anyone approaching beyond the safe position and operate a stop or reverse control viz. trip rod, hand trip on a rubber mill, electronic eye, platen printing guard, and dough brakes, etc.
- Control guard:
- The control guard activates the machine when the guard is closed and opening the guard stops it. Thus, the guard acts as an on-off switch. Such guard is useful for the machine which can be brought to rest rapidly.
- Very flexible to accommodate various types of stock.
- Manually adjusted.
- The opening is determined by the movement of the stock through the guard.
- Does not always provide maximum protection.
- Common complaint- reduced visibility at the point of operation.
Principles of machine guarding :
Elimination of hazard:
The basic steps to prevent accidents are:
- Eliminate the hazard from the machine, material, method, and structure, etc.
- Control the hazard by enclosing or guarding it at its source.
- Train personnel to know the hazard and to follow the safe job methods to avoid it.
- Use personal protective equipment necessary
Industrial machines, including many special machines that are available, may be classified as follows:
- Forming, punching, and shearing machines, including power and hand presses, forging hammers, power brakes, and shears.
- Milling, drilling, and grinding machines, including drill presses, boring mills, lathes, shapers, and grinders.
- Sawing, ripping, and planning machines, including circular and band saws, jointers, and planers.
These machines have the following sources of hazards:
- Unguarded points of operation, such as the punch and die of a power press, the cutting edge of a power shear, and the circular blade of a power saw.
- Unguarded belts, chain drives, and gears, such as the belt drive on a drill press and the gears of a lathe (power-transmission component).
- Unguarded spindles, shafts, and flywheels, such as the spindle of a drill press and the flywheel of a power brake.
- There are three factors that have established safe machine design (guarding) as an absolute must. Machine purchasers are demanding that the machines and tools they buy meet the standard Acts, local, state, and company safety standards, particularly the machine-guarding standards and the noise standard. Machine manufacturers are compelled by competition to develop and produce a safe machine or tool, and the growing threat of product liability is literally forcing machine builders to produce the safest kind of product.
- Machine designers and safety engineers have the know-how and expertise to make old machines safe and to produce new equipment, tools, and devices that meet standards and company safety requirements.
- Part of this task is relatively simple. Guarding belts, pulleys, gears, and other power transmission parts is no problem. But protecting the operator at the point of operation is more complicated.
A good rule to remember is:
- Any machine part, function, or process which may cause injury must be safeguarded. Where the operation of a machine or accidental contact with it can injure the operator or others in the vicinity, the hazards must be either controlled or eliminated.
The thrust areas are:
- Where mechanical hazards occur.
- The hazards created by different kinds of motions.
- The requirements for effective safeguards.
- Describe the basic hazards involving machinery including point of operation and power transmission devices.
- Introduce control measures through effective machine guarding principles and methods.
What is an Automatic guard?
Automatic guards ensure that the operator is prevented from coming in contact with dangerous parts when the machine is in motion. This guard is itself actuated by the movement of the dangerous part. It can only be used where sufficient time is available to remove the body part from the danger zone viz. sweep, knock or push away guards on a power press or platen printing machine. Because of its stroke hazard, it is hardly used. Photoelectric guard (device0 operates to stop the dangerous part when the light curtain is interrupted.
which of the following machine parts always require guards?
- Dangerous moving parts in three basic areas require machine guarding:
1. The point of operation
- That point where work is performed.
2. Power transmission apparatus
- All components of the mechanical system transmit energy to the part of the machine performing the work.
3.Other moving parts
- All parts of the machine moves while the machine is working.
- Feed mechanisms
- Auxiliary parts
Hazardous Mechanical Motions & Actions
- A wide variety of mechanical motions and actions may present hazards to the worker.
- These can include the movement of
- Rotating members,
- Reciprocating arms,
- Moving belts,
- Meshing gears,
- Cutting teeth, and
- Any parts that impact or shear.
- These different types of hazardous mechanical motions and actions are basic in varying -combinations to nearly all machines, and recognizing them is the first step toward protecting workers from the danger they present.
Machine safety checklist:
- Design the machine so that it is impossible for an operator to get at the point of operation or any other hazard point while the machine is working.
- Design the machine so that corners and edges are rounded.
- Locate machine controls so that the operator will not be in the vicinity of the point of operation while actuating the controls.
- Place the control so that the operator will not have to reach too far or move his body off balance in order to operate the machine.
- Build power transmission and drive mechanisms an integral part of the machine.
- Build overload devices into the machine.
- Design the machine for single-point lubrication.
- Design mechanical, instead of manual holding devices.
- Design mechanical devices for feeding & ejecting parts so as to eliminate the use of hands for such operations.
- Minimize motor drift time.
- Provide failsafe interlocks so that the machine can not be started when it is being loaded or unloaded or being worked on.
- Provide a grounding system for all electrical equipment.
- Provide standard access platform and ladders for inspection and maintenance of equipment.
- Design component part of equipment for easy and safe removal and placement to facilitate maintenance and
- Reduce sources of excessive noise, vibration, heat, etc.
- such built-in care in construction principles of a machine ensures that no harm to the environment & no discomfort/accident to the operator.
Machine Guarding terminology:
Any means of effectively preventing personnel from coming in contact with the moving parts of machinery or equipment could cause physical harm to the personnel. The word ‘guard’ is exclusively used for barriers designed for safeguarding at the point of operation. The word ‘safeguard’ should be used for a barrier or cover that protects other danger zones.
Guarding by fixed physical barriers that are mounted on a machine to prevent access to the moving parts. They are most effective when they are designed as an integral part of the machine, but they can be bolted or welded to the frame.
Guarding by means of a fence or rail enclosure restricts access to a machine except by authorized personnel. Such enclosures must be a minimum of 42 in. away from the dangerous part of a machine.
Guarding is the result of the physical inaccessibility of a particular hazard under normal operating conditions or use.
- Point of operation:
That area of a machine where the material is positioned for processing or change by the machine.
- Power transmission:
All mechanical components including gears, cams, shafts, pulleys, belts, and rods which transmit energy and motion from the source of power to the point of operation.
- Ingoing nip points or bites:
A hazard area is created by two or more mechanical components rotating in opposite directions in the same plane and in close conjunction and interaction.
- Shear points:
A hazard area is created by a reciprocal (sliding) movement past a stationary point on a machine.
Guards prevent injuries from the following sources:
- Direct contact with exposed moving parts of a machine, either point of operation on production machines like power presses, machine tools, or woodworking equipment or power transmitting parts of mechanisms like gears, pulleys, and sheaves, slides, or couplings, etc.
- Work in process, for example, pieces of wood that kickback from a power ripsaw, or metal chips that fly from tools or from abrasive wheels.
- Machine failure, which usually results from lack of preventive maintenance, overloading, metal fatigue, or abuse.
- Electrical failure, which may cause malfunctioning of the machine or electrical shocks or burns.
- Operator error or human failure caused by lack of knowledge or skill, also by emotional distractions, misunderstandings, laziness, unsafe operation, illness, fatigue, and so on.
Need and Importance of machine guarding:
The basic need of machine guarding is to protect against contact with the dangerous and moving parts of a machine, work in process, and failure due to mechanical, electrical, chemical, or human causes. The guards remove worker’s fear and thereby increase production. They allow the operation at higher speeds and compensate for the expenditure on guarding.
Groups of dangerous parts:
(A) According to motions:
Group-1. Rotary Motions:
- Rotating parts alone viz. shafts, couplings, spindles, etc.
- In-running nips subdivided as:
- Between parts rotating in opposite direction – gears, rolls, etc.
- Between rotating and tangentially moving parts – conveyors, belt drives, etc.
- Between rotating and fixed parts – grinding wheel, paper machine felt or roll, etc.
Reciprocating Sliding Motions:
- Group-2: Reciprocating and sliding motions and fixed parts:
- Approach type danger of crushing, viz. slides (rams) on power presses and forging hammers, pistons, cross rod of a steam engine and revolving machines.
- Passing types-danger of shearing, viz. planning machine, shaper, spot welder clamping fixtures, guillotine and the shear, power presses etc.
- Single sliding motion – abrasive or sharp nature of objects such as saws or crocodile clips on belts.
- Group-3. Rotating/sliding Motion: A cam gear having sliding and turning movement etc. fall within this group.
- Group-4. Oscillating motions: Trapping points between two moving parts or between a moving part and a fixed object viz. a pendulum, crankshaft, etc.
B) According to H.A.Hepburn:
(a) Single Revolving Units – Risk of Entanglement
- Revolving open arm pulleys and other discontinuous rotating parts – fan blades, spur gear wheels, etc.
- Revolving worms or spirals in castings – Meat mixers, rubber extruders, spiral conveyors.
- Projections on revolving parts – key heads, set screws, cotter pins, coupling belts, etc.
- Revolving shafts, spindles, mandrels, and bars – drills, reamers, boring bar, stock bar, milling, etc.
b) Single Revolving Units – Risk of cutting or abrading:
- Revolving high-speed cages in castings – Hydro-extractors, centrifuge.
- Revolving or oscillating mixer arms in castings – Dough mixture, rubber solution mixture.
- Revolving drums and cylinders, uncased- ramblers, shaking barrels, rag digesters, etc.
- Revolving cutting tools – circular saws, milling cutters, shears, routers, chain mortises.
- Abrasive wheels grinding wheels etc.
c) In running parts – Risk of nipping and tearing:
- In running nips of the belt and pulley type belt, chain and sprocket gear, conveyer belt, etc.
- Revolving beaters, spiked cylinders and drum scrubbers, cotton opener, laundry washers.
- In running nip between pairs of revolving parts, gear wheels, friction wheels, calendar bowls, mangle rolls, etc.
- Nips between gears and rack strips.
d) Reciprocating Tools or Parts – Risk of cutting or crushing
- Moving balance weights and dead weights – Hydraulic accumulators, balance weight on the slotting machine, etc.
- Reciprocating knives and saws – guillotines for metal, rubber, and paper cutting, trimmers, perforators, etc.
- Nips between reciprocating and fixed parts other than tools and die, sliding table and fixture, shaping machine and fixture.
- Closing nips between platen motions – letterpress platen printing machine, power presses.
- Reciprocating tools and dies – power presses, drop stamps, relief stamps, bending press, revolution press.
- Nips between revolving control handles and fixed part – traverse gear handles of lathes, milling machine, etc.
- Nips between revolving wheels or cylinders in pans or on tables – sand mixtures, edge runners, crushing mill, mortar mill, etc.
- Nips between fixed parts and unidirectional moving parts – buckets or hoppers or conveyors against tipping bars, etc.
- Nips between connecting rods or links and rotating wheels, cranks or disc – flatbed printing machine, jacquard loom, automatic looms.
- Pawl and notched wheel devices for intermittent feed motions – planer tool feed motion, power press dial feed table, etc.
(e) Running Edges – Risk of cutting
- Cutting edges of endless band cutting machines – band saws, band knives.
- Projecting belt fasteners and fast running belts – bolt and nut fasteners, wire pin fasteners, centrifuge belts, etc.
Design principles of machine guarding:
- Guards should be convenient, reliable and should not hamper production.
- It should be a complete guard and must satisfy the legal provisions and Standards.
- It should provide strength to the machine.
- It should allow for oiling, inspection, adjustment, and for repair.
- It should withstand wear, shock, vibration, and long use with minimum maintenance.
- It should be of proper material and free from self hazards.
- It should fail-safe and be an interlocking type.
Ergonomics of Machine Guarding:
- Ergonomics is the studying, measuring, and organizing the work. The focus of the study is the worker interacting with the engineering environment. The complexity of human nature and its limitations are considered while designing any machine or equipment like guards etc.
- ‘Ergon’ means work and ‘nomos’ means habits or law. Thus ergonomics principles include comfortable posture, balanced and easy motions.
- Ergonomist is expected to take an overall view and identify the key design aspects for particular people engaged in particular tasks.
Aims of Ergonomics:
- To facilitate a person to do a job efficiently.
- To minimize the possibility of human error or mistake.
- To develop knowledge and techniques by using combined experiences and systems approach.
- To solve human problems of work performance by considering human factors in the design of machines and work and safe job.
Human factors in the Design of machines and Work:
- Increased stress.
- Uninterested work.
- Errors and accidents.
- Work-related diseases.
- Awkward body position causing difficulty.
- Confusing signals causing human errors in a hurry.
- Repetitive tasks producing boredom and tiredness.
- Bent or awkward posture while handling heavy materials and causing back pain.
- Perceptual limitation.
- Short-term memory.
- Incompatibility in a man-machine relationship.
- Risk or chance taking.
- Position or location not suitable to a worker such as unsuitable work height, seat, the layout of control, and displays.
- Organization of heavy work. And
Basic requirements of a Guard:
- With its primary purpose of protection, it should also facilitate the work i.e. it should be convenient, reliable, and not hampering the work or production.
- It should fully satisfy the legal provisions and Is prescribed i.e. it should be a complete guard. It should be as close as possible.
- It should be suitable and effective for the job and the machine.
- It should allow for oiling, inspection, adjustment, and repair. if it requires opening for this purpose, it should be easily and quickly replaceable.
- It should withstand wear, shock, vibration, and long use with minimum maintenance. If it requires frequent opening and closing, this factor becomes more important.
- It should be of proper material and construction. Fire and corrosion-resistant material are preferable.
- It should be free from self-hazard such as sharp or rough edges, nails, etc.
- It should be transparent and durable if a visual watch of operation is necessary.
- In a dusty atmosphere, a dust suction device should also be provided as a special guarding.
- It should be fail-safe i.e. if it fails or breaks it should either stop the machine or give the warning to stop the machine.
- It should be interlocking type i.e. the machine will not start till it is not closed and will stop soon if it is opened.
- It should fulfill special requirements depending upon its purpose viz. distance guard should provide sufficient protective distance, the trip guard must immediately trip the machine, etc.
Total machine safety
- Safety by construction and in-built safety.
- Safety by position.
- Safety by guarding.
- Safety by machine controls.
- Safety by precautions and maintenance.
Safety by Construction:
- It indicates parts so constructed as to cause no hazard, viz. shaft, sliding, and link mechanism so located or with such speed that their contact is not dangerous.
- Built-in-safety is a similar word for designing and constructing new machinery in such a way to make the dangerous parts safe by deep housing or position etc.
Safety by position:
- It means the situation or position of dangerous parts should be kept in such a way that it will not possible to touch them, but intentional contact should be prevented.
- Moving feed opening can be made safe by position if gravity or remote feeding device is applied.
Safety by Guarding:
- A guard should be selected depending upon the dangerous part, its size, position, speed, etc.
- Parts to be guarded fall within 3 categories:
- The prime mover.
- Transmission parts from the prime mover to the machine & transmission parts in the machine itself.
- Points of dangerous operation need effective guarding.
Safety by machine controls:
- The machine itself has many controls as its inherent or attached elements. All levers, buttons, brakes, pedals, switches, handles, wheels, auto controls, etc. are machine controls. Such controls should be clearly identified and suitably placed for ease of operation. Their purpose and direction and direction of movement shall be marked clearly for ease of operation.
Safety by precautions and maintenance:
- All precautionary operating measures are necessary for addition to the machine guarding. The machine operators should be made aware of hazards in their work, location and operation of machine controls, regular checking of guards, warning for guards and repairs, adjustments, etc. by specially trained persons.
- The need of wearing tight-fitting clothing, the use of proper Personal Protective Equipment, and the use of the right tools and equipment are essential.
What is Indirect machine guarding?
- Two hand control: Two push buttons are required to be operated simultaneously thus keeping the operators’ hands away from the dangerous parts.
- Optical sensor: this uses a photoelectric cell and light.
- Electromagnetic Sensor.
- Mechanical feed.
- Feed tools pedal guards
- Feed hopper etc.
Materials for machine guard construction
- Generally, metal is preferred for machine guard construction. Bar, pipe, rod are used for the structural frame. The filler material may be solid, perforated, or expanded sheet metal or wire mesh, where transparency is necessary plastic or glass material can be used. Safety glass and plastics are also used to protect against flying particles and chips.
- Wood guards have limited application due to their poor strength and durability, flammability, and high maintenance cost.
- Aluminum guards are preferred where rusting or damage to tools or machinery are considerable.
- When a guard could not be made to exclude lint, fume, dusting, etc, extra ventilation should be provided.
- large guards should have self-closing doors for cleaning. Surrounding space should be provided for repair and maintenance.
- The guard color may be red or the same color of the machine is a matter of choice.
Maintenance and repairs of guards:
- Regular inspection of guards can detect the guard removal, damage, or requiring repairs.
- A guard removed for repair should be replaced at the earliest possible and meanwhile, the temporary guard should be installed.
- Good maintenance increases the life of guards and vice-versa.
- The supervisor should pay constant attention to this.