The early development of this type of conveyors is reputed to have originated in the pork packing houses in the mid-west of
The basic constituents remain namely:
• The track: This usually is a rolled steel hoist of size varying from approximately 75 x 40 mm to 150 x 75 mm with popular standard 100 x 75 mm for loads of around 350kg/single trolley.
• The chain: For the standard, most frequently of forged links and pins or of plate link type of internationally acceptable 458 pattern with a pitch of approximately 100 mm and breaking strain in region of 20 tones.
• The drive unit: Either the sprocket type or more modern eater-pillar type which has advantage that it can be inserted at almost any point in track as required by pull in the chain and does not involve diverting the line of the conveyor to accommodate a sprocket.
Twin Track Conveyors
The twin track or 'Dual duty' conveyors were developed during 1930's by Donald. M. King. In this application the loads were no longer attached directly to endless conveyor chain but where suspended from four wheel trolleys running on a parallel track below the chain track. The chain track was fitted with hinged dogs capable of driving or retaining load trolleys this was a great advance for the loads could now be diverted from main track onto sidings or Dead line where painting or inspection operations could be undertaken or could be held for storage. The full advantages of this new system were not utilized for some 20 years, due mainly to war-time activities, but in 1950 further development permitted the dead lines to be driven as separate conveyors.
An overhead conveyor is particularly suited to the provision of loading and unloading devices which utilizes the motion of conveyor for their operation. These often incorporate roller conveyor, ramps or air-operated rams, for diversion of the loads and can be made selective by limit switches actuated by pegs on suspension members on the carriers for load.
Floor Mounted Chain Conveyors
This type of conveyor with its twin chain and wood or metal slats falls into two categories.
Ø The service conveyors: This is used for moving goods from one position to another in a straight line in a level or inclined plane.
Ø The process conveyors: This application is arranged so that work may be carried out on goods while they are in motion.
The service conveyor is much simpler and straight forward; chains are driven by two, usually octagonal, sprockets mounted on a common shaft driven by a worm or train of spur gears, the final drive being roller chain in most cases to ensure compactness. The material of slats is normally dictated by duty requirements. A tractive effort for this type of conveyors is usually around 5%. The structural member consists of angles for track supported on intermediate channels forming legs to suit the application. The advantage of the slat over the belt conveyor for this type of use is primarily the rigidity of the continuous working surface which can support jigs and fixtures and is readily adaptable to the incorporation of automatic processes. With frames of appropriate height, the slats can become a travelling work bench. The height can be varied at appropriate locations along run of conveyors as in case of cake stand conveyors of motor car assembly plants. Here the car frames and transmission unit are set up on a single slat conveyor and when the wheels and tires are added they are easily transferred to a twin slat line were chocks act on the car wheels up and inclined to a higher level. The vehicle is now on the cake stand track where the operators can work on the platforms carried by the conveyor structure or inside the car or beneath it from the ground level. Care is required on such conveyors to ensure that the chock referred to above are protected where they descend below ground level in order to avoid workers feet being trapped. For flat ground level conveyors run carrying wheeled vehicles. The level of slats at pre-arranged pitches can be lowered so that vehicles are duly spaced.
As an alternative to beach type conveyors a more recent development is for slats to run at floor level and support work-carrying pedestals. This enables the operator to have all round access to his job and when necessary. The head of work carrier can provide a swivel unit.
· Floor conveyors without slats:
When it is required to convey pallets, work carrying skids or fixtures, slats are not needed and carrying unit is mounted directly on chain. Alternatively, if wheeled trucks are involved, dogs are provided on chain to engage the abutments on the trucks. This provides two further classifications namely double strand floor conveyors and drag link floor conveyors.
· Double strand conveyors
These could be also described as slat conveyors without slats. The chains are often of the raised link type where rollers provided at each end of links are offset to ensure that the links neither drag on the track nor rub on loads. This greatly reduces the chain pull, allows for longer length and smoother running on Drag link conveyors.
Drag link conveyors are normally of single strand chain often of the 458 or 678 pattern as used for overhead conveyors, mounted on floor in a box type truck with guide channel rails for trucks. The conveyors are ideal for paint plants because, there is no chance of dust or grease being dropped from an overhead track. The trucks however must be accurately made and constantly maintained and checked particularly where automatic painting is involved.
· Floor conveyors (carrousel)
This type of conveyors is frequently used for store services and small part assembly. The layout is a continuous loop with shaped slats driven by the chain mounted centrally. The slats may require overlapping to prevent screws and small work pieces falling between them which would result in jamming on bends.
Belt Conveyors
There are two classes of belt conveyors, flat belt and troughed belt. Flat belt is used for unit handling and troughed belt type for bulk handling. Both are driven from the far-end by a head-drum usually crowned to minimize the danger of belt tracking or drifting to one side. Self aligning roller sets are also available to assist tracking for long conveyors. The calculation for drive unit is based on normal belt drive formula and demands the determination of effective tension required to more the empty belt, to move the load and to raise or lower the load. Belts are normally rubber or plastic covered textile material, ranging from cotton, doubled cotton/nylon, rayon, rayon/nylon to all terylene and terylene / nylon of varying plies. The covering is obtainable in differing thickness as demanded by the duty. Rubber tired idler rollers are also obtainable to suit impact duty. Belting of stainless steel is frequently used in food industry and woven wire belts are normal where high temp service is required.
Belt conveyors have a good record of trouble free running even though high speeds some time in excess of 3m/s are common with high long gradients, and heavy and sharp materials are dealt with. The electrical equipment is usually quite straight forward but interlocks should be provided in any continuous run of conveyors to ensure that if any one of these stops, or develops serious belt slips, the whole series will stop together with any automatic loading mechanism.
Troughed belt, however, preset a more difficult problem, which can be solved by a tipper. This device incorporates a gradually inclined track with troughed idler pulleys. The conveyor belt is carried up a ramp and over a drum at a highest point so that material being conveyed tips onto a built in chute. The built is then fed over a further drum at conveyor level and hence to its normal operating position. Trippers are mounted on four rails, wheels, fitted with locking device to retain them in positions. The installation of belt conveyors is extremely important, particularly of long runs. The drive and tension units must be carefully aligned and all idlers positioned and set up accurately if good belt tracking is to be assured.
Pneumatic Conveyors
Pneumatic ducting can be routed along walls and ceilings and enable saving of floors space. Operators can transmit materials between in accessible points without radical structural alterations to existing premises.
Bulk purchasing is made practicable for smaller firms, with resultant saving of expense.
Improved house keeping, Pneumatic handling virtually eliminates wastage and spillage and is clean and economical.
Lower running cost. Few working parts and involved and reduced maintenance costs are thus achieved.
Positive overall plant efficiency over more usually methods of bulk handling can be achieved.
Many different systems are available. The simple pressure method with a blower pack induces the pressure into the ducting, the material to be conveyed being fed from hopper via a rotary valve. The final storage bin is fitted with filters to clean the exhaust air. The simple vacuum system uses a similar layout except that no rotary valve is required and the blower pack is fitted at the end of the line connected to the filter on the storage bin. The air involved can be then recycled. Various combinations of the two systems are built up to suite the particular requirements. It is important that the purchaser should ensure that the supplier has the facility to run test of samples of this material to make sure that this system is the best for the particular material and no damage will be done to the material in this rotary valves etc.
Pneumatic conveyor systems lend themselves quite easily to automation and process control. As a bonus it is frequently possible to arrange for actual processing during conveying.
Level control in hoppers has been a problem. This has been overcome by use of rotating paddle controllers which are insensitive to dust and pressure but give positive control. It can be expected that this type of bulk handling will be increasingly used for heavier work such as the conveying of pulverized fuel, for quarry work and in iron and steel industry.
Screw and 'En Masse' Conveyors
These conveyors can be used for either horizontal or vertical transport or run under mill bore condition. The screw conveyor which has been called an animated mincing machine certainly employs the same principle. The 'En Masse' type conveyor consists of a special endless chain with H-shaped attachments at frequent pitches housed in rectangular ducting. The forces exerted by the attachments and the chain exceed the friction force on material on the sides of the housing.
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