Machine Guarding

Machine Guarding


  • It is first line of defense against hazards of machine operations.
  • It protects against caught in between, nip points, flying objects, rotating parts, sparks, cuts, etc.
  • It provides maximum protection with low maintenance.

Principal of Machine Guarding

  • The main safety principle of machine guarding is that it must permit safe operation without risk to the health of the operator.
  • The most effective and economical way to achieve this is to incorporate safe guard into initial design of the machine (Design out hazards) factor such as speed, temperature, pressure should be reduced to the lowest level.
  • The guard must not interfere with the operation of the machine, resulting in reduction of production. This affects the incentive of worker. Making him remove or bypass the guard.
  • The guard assist and improve of output of the machine while at the same time provide require level of protection .

Types of Machine Guarding

  • Fixed.
    It is fixed part attached to machine and can`t move while machine is in use.
    It is used to enclose operation going on and to protect workmen from operation.
    It needs to remove in case of any maintenance of Machine.
    Examples : Belts, Pulleys, Etc.
  • Interlocked.
    It shut off machine or cut off energy in case of it`s removal to protect workmen from machine hazard.
    It is also known as Barrier Guarding.
    It allows safe access to inner parts of the machine.
    It requires careful operation and maintenance, It is dangerous in case of bypassing it.
    Types of Interlocking Guards:
    1. Mechanical interlocking.
    2. Electro-mechanical interlocking.
    3. Relay interlocking.
    4. Electronic interlocking.
    Examples : Lever, Brake Pedals, etc.
  • Adjustable.
    It is like fixed guard but can be adjustable as per requirement.
    It can cause hazards if not adjusted properly. So requires trained workmen to operate machines with this type of guarding.
    Examples : Band saw adjusting guard, Adjustment Knob, Etc.
  • Self-Adjusting.
    It is more or less as same as adjustable guards, but it automatically adjust as per requirement.
    It allows workmen to feed material in machine just enough with maximum protection.
    Examples : Band Saw, Wood Cutting Machine, Packing Machines, etc.

Ergonomics of machine guarding

  • Ergonomics is a useful tool for Fitting work to people, not people to work. ‘Ergon’ means work & Nomos’ means habit of law.
  • Thus ergonomics is for the purpose of solving the problems of work habits.
  • It requires designing of proper workplace, working posture, reach, seat, control, knob, brake, display, communication, skills, work intensity, sufficient time for rest, good environment, clear understanding, & providing platforms or lift table, carts, transport ways, proper storage racks, buffer stocks, good lighting, ventilation & housekeeping, proper guards,& good welfare facilities.
  • Thus ergonomic principles include comfortable posture, balanced & easy motions, adequate rest & enough variety. 

Design of Guards

  • Guards on machine are intended to keep people and their clothing from coming into contact with hazardous parts of the machine and equipment.
  • They also prevent flying particles and broken machine part from coming into contact with striking people. Guards also enclose noise or dust from part of the exhaust system.

Design Principle of Guard

  • Basic requirement or good guarding practice for machine guarding are :
  • With its primary purpose of protection, it should  also facilitate work, it should be convenient, reliable &  not hampering the work or rate of production.
  • It should fully satisfy the legal provisions & IS prescription i.e. it should conform the standards, be a complete guard & not incomplete or giving any accessible to protect the part. It should be as close as possible.
  • It should be suitable & effective to the job & the machine. It should not weaken the machine.
  • It should allow for oiling, inspection, adjustment & repair. If requires opening for this purpose, it should be easily & quickly replaceable.
  • It should withstand wear, shock, vibration & long use with minimum maintenance. If it requires frequent opening & closing, this factor become more important. 
  • It should be of proper materials & construction with good fitting. Fire & corrosion resistant materials is preferable.
  • It should be free from self hazard such as sharp or rough edges, nails, splinters, more opening, noise, vibration.
  • It visual watch of operation is necessary, it should be transparent & yet durable.
  • If dusting is possible as in case of machining of wood, rubber, brass, cast iron etc., apart from the guard , fit dust suction device as a special guarding.
  • It should be fail-safe i.e. if it fails or breaks it should stop the machine.
  • It should be interlocking type i.e. the machine will not stop till it is not closed & will stop soon if it is opened.
  • It should fulfill special requirement depending upon its purpose.
  • It may be difficult to fulfill all these requirements but it is desirable that safety officers must design guards by keeping above points in mind.  

Elimination of Hazard

  • The basic step to prevent accidents are;
  • Elimination the hazard from the machine, method, materials, structure etc.
  • Control the hazard by enclosing or guarding it at its source.
  • Train personnel to know that hazard & to follow the safe job method to avoid it &
  • Use personnel protective equipments necessary.
  • Thus machine guarding is one of the basic step to eliminate hazard. Actually design and construction of the machine as such that incorporation of all safety points by built in safety principle & the there is need of extra guards is minimum.

Machine Safety check list

  • Design the machine so that it is impossible for an operator to get the point of operation or any other hazard point while the machine is working.
  • Design the machine which has round corners & edges.
  • Locate machine controls so that the operator will not be in the vicinity of the point of operation while actuating the control.
  • 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 & drive mechanisms as integral parts of the machine.
  • Build overload device into the machine.
  • Design mechanical instead of manual holding.
  • Design a mechanical device for feeding & ejecting parts so as to eliminate the use of hands for such operation.
  • Minimize motor draft-time.
  • Provide fail-safe interlocks so that the machine will not start when it is being loading or unloading or being in operation.
  • Provide a grounding system for all electrical equipment.
  • Provide standard access platform & ladders for inspection & maintenance of equipment.
  • Reduce sources of excessive noise, vibration, heat etc.
  • Such built-in-care in construction principles of a machine or equipment should also ensure that it will cause no harm to the environment, no discomfort to the operator, no operational contingencies, no contact with overheated or chilled  surfaces, no electrical accident & no access to the danger zone.

Material of Guard construction

  • Sheet Metals:
    Use of sheet metal in construction of guard.
    It is not suitable where ventilation is necessary to keep the machine part from overheating.
  • Welded Mesh:
    Welded mesh has considerable strength.
    We can change shape as well.
    It permits viewing of work inside the guard.
    It is not suitable to use chicken wire and woven wire as fencing materials as they do not have necessary strength & trend to get distort.
  •  Use Expanded or perforated metal of suitable gauge to making guards. They are mounting on a frame to give support. It s finished to remove burns.
  • Polycarbonate Sheets.
    Polycarbonate & plastic material easy to form with highly impact resistance are soft.
    They are not scratch proof.
    They attract dust.
    They afford clean visibility of th work.
  • Perspex: Perspex is hard, brittle & plastic. It shatters when struck. It is less suitable then polycarbonate.
  • Timber & Hard Boards :
    They are easy to manipulate.
    It is of rigid structures.
    It is with limited life.
    They are flammable & should used where fire risk is minimum.
  • Use of molded Shaped thermostatic plastic into any profile.
    Commonly used in manufacture of guards for manually operated tools.

Machine Hazard

There are five types of machine motions that create machine hazards.

  • Rotation Hazards :
    Spindles, chucks, drill bits, shafts, splines, coupling, flywheels rotate to perform a job. Rough surfaces & rotary motion tend to catch clothing of any person in the vicinity & pull into the machine. Skin contact with any rotating component can cause serious injury.
  • Reciprocating Hazards :
    Back & forth motion ( vertical & Horizontal) is known as reciprocating motion.
    Any object is line of motion can be struck by moving part of the reciprocating machine.
  • In running points :
    Machine parts that rotate towards each other create nip point, or pinch at the point of contact of the two. The nip point can cause severe injury.
    Example  of running nip points are,
    a. Belt & Pulley.
    b. Chain & Sprocket.
    c. Gears.
    d. Conveyor belt & rollers.
  • Cutting action :
    machine remove & cut materials by means of cutting action.
    Examples of cutting machines,
    Saw, Milling, Shaping, Planning, Lathe, Grinder, Boring machines.
    The point where the cutting takes place is known as point of operation & is a hazardous area.
  • Punching, shearing, & bending motion :
    In this type of motion, two machine parts come together to perform a job.
    Example of these are :
    Punching machine, Shearing, Shaping, Drawing  Trimming machine.

Types of Safe guards

Safe guards include,

  • Guards :
    Guards are barriers that prevent the entry of a person’s body & clothing into a hazardous part of the machine. They also prevent materials striking & causing injury.
  • Devices :
    Device are controls or attachments that stop normal operation of the machine if any part of a person’ body is within the hazardous area.
  • Distance :
    Distance place hazardous area of the machine vertically, or horizontally out of reach to prevent any contact with access to dangerous parts.
  • Location :
    Placing hazardous machine or components where people are not usually there. 

Built-in-Safety devices

  • Machine guarding lead to better accident prevention, with less discomfort for the operator & also provide protection for ay approaching the machine.
  • But many of theses guards designed & mounted on the machine after manufacture could be easily removed if they interfered with production.
  • So provision of guard was not fool proof. Hence reconsideration of the guarding of the machine from a different angle where ever possible. That is from ergonomics & man/machine interface point of view.
  • So we can avoid accident hazard but rendering impossible. This is known as built in safety or fool proof system. 

Maintenance of & Repair of Guard

  • As per legal requirement, the guards shall be constantly maintained & kept in position. Such maintenance is obviously necessary because a machine running without guards or with open or broken guards pose danger.
  • Regular inspection of guard can detect the guard removed, damaged or requiring repairs.
  • Removal of a guard for repair needs to fit at the earliest possible & meanwhile there is instalation of temporary guard or stop the machine.
  • Good maintenance increase life of guards & vice versa.
  • Designing or fitting of a guard as such that there is no requirement to open for oiling/ greasing.
  • The part needing regular service should be located outside the guard.
  • Automatic controls for lubrication, adjustment or service can be provided by a sophisticated equipment.
  • Interlock type guard ensures that the machine can not be able to operate unless all guards are in position. Such planning help maintenance without disturbing guards.

Advantage of built in Safeguard

  • Built in safe guards conform more closely to the contours of the machines, thus making them superior in appearance.
  •  It eliminate hazards completely & permanent while withstanding daily wear & handling. Most makeshift safeguard by contrast little protection against mechanism against human failure. Yet they may give operator false sense of security & thus may be more harmful then no safeguard at all. To make up for their inadequacy. Operator must be constantly alert, which can make work stress & fatiguing. 
  • The guard must assist and improve the output of the machine while at the same time provide require of protection.
  • Guards are compatible with taught and instructed way of operation of the machines.
  • The guard should allow the operator to work at his natural and most effective rate.

Zero Mechanical state

  • Primary source of power of all machines is mostly electricity. In some cases use of pneumatic , hydraulic & charged capacitors to execute the work, when the equipment is first turned on after connecting the primary source of power.
  • Charging of Pneumatic, hydraulic components before the machine can perform properly. This may be in the form of pumping the accumulation & surge tanks, cylinders loading hoppers, lifting counter weights, all these action result in energy of one form or another stored in parts of the machine.
  • Shutting down the equipment should begin with procedures for turning all operation controls to OFF position.
  • Next disconnect all energy sources to the equipment to lock out.
  • Make accumulators to atmospheric pressure.
  • Down pneumatic pressure to nil.
  • Provide earthing to charged capacitors & counter weight to ground level.
  • In the stored kinetic & potential energy of a system is not brought to neutral condition & place the in zero Mechanical State (ZMS) accident may occur due to sudden release of stored energy. 

Lock out and Tag out

  • A machines & equipment have parts that wear out, components that need periodic adjustments or places that have to be serviced for one reason or other. When such event occurs, the machinery is shut down for maintenance work.
  • During maintenance remove the protective guards to obtain access to unserviceable parts, pass interlock , disconnect safety device.
  • During maintenance some workers may have to put portion of them body into hazardous area, protect them by the inoperative safety device.
  • If starting of the machines at the juncture by somebody who does not know the worker in inside the machine serious accident can occur.
  • To prevent such an occurrence develop lockout or tag out procedure require the worker to secure the equipment in such manner that it will not start by any other person than himself.
  • The feature common to there devices is a pair of mating tongs which have matching holes through which a restraining device can be placed.
  • The restraining device may have several pairs of hole, through which individual workers involved in the maintenance work place their personal locks to which they hold the key.
  • In practice individual worker who is involved in the maintenance work places his individual locks on the lock out device before entry into the machine and work.
  • The lock cannot be removed by anyone else until the worker has completed his assignment task and return to lock out location.
  • In this way it ensured that all workers are out of the machine before that it is switched on.
  • Tag out device is similar lock out but the involves the application of durable tag rather than locks.
  • There are attached by a non- reversible link and sealable to prevent reuse.
  • Such tags bear a legend such as ‘DO NOT START’.
  • The link is cut and the tag is removed by worker when the task is completed. This system is not as secure as lock out. 

Guards must have certain characteristic

  • Permanent part of the machine.
  • Must prevent access to danger zone during operation.
  • Must be durable and strong enough to resist wear and abuse and environment.
  • They must not interfere with operation of the machine.
  • They should protect workers during inspection, adjustments, lubrication, cleaning and repair.
  • All maintenance activities should be able to be carried out without removing them.
  • They should be reliable and function without fail under various operation condition over a period of time.
  • They should operate precisely.
  • Safety mechanism should be correctly assembled.
  • It should be fail safe. If machine fails, it must do so in such a way that the machine should stop and guard stays enclosed and not vice versa.

Common Advantages of Machine Guarding

  • Possible to construct in plant.
  • Provide maximum protection.
  • Suitable for high production and repetitive operations.
  • Requires minimum maintenance.
  • Commercial availability in the market.
  • Flexibility.

Limitations or Disadvantages of Machine Guarding

  • It does not always provide maximum protection, it depends on adjustment, maintenance, location, use, etc.
  • It may interfere visibility.
  • It may require frequent maintenance, adjustment and repairing.
  • It may be bypassed by workmen.
  • It has limitation to specific operations.
  • It is easy to disengage.