Problems of Conveyor Controls

General. Application of electrical controls for operations of conveyors poses problems which are not present in the control of precision machines. Even large transfer machines, while consisting of a large number of parts, will occupy relatively small, compact areas. Conveyors often operate over very large areas. Individual conveyors are seldom less than 100 ft and are frequently 2000 to 10,000 ft or more long. Conveyor systems may extend into several buildings and on more than one floor. Large portions of systems may be overhead in otherwise unused space and are relatively inaccessible.

Conveyors are seldom assembled and tested except in their permanent location. Once a conveyor is installed, the plant must get into production quickly so there is a minimum opportunity to make changes or adjustments. This requires careful engineering to ensure immediate operation. Frequently adjustments must be made at expensive overtime costs. Mechanical precision is not a general characteristic of conveyors. Basic designs have been fixed by usage and proved adequate before the advent of special controls now added. Many conveyors are designed around drop-forged rivetless chains. These chains have least weight and cost for their strength. Most are heat treated while the larger sizes may be made of alloy steel for greater strength.

Few chains are made to precise pitch. Dimensions of drop forged chain change slightly as forgings and trim dies wear. Normal runout is about 1 %, in. per 10 ft of #458 chain, the most commonly used size. Rough spots wear down quite rapidly during first weeks of operation. Then wear and elongation changes remain about constant at a slower rate. Chains can elongate as much as 5% before they need replacing. Attachments for loads can normally be spaced only at multiples of twice the chain pitch. Special provisions may be made for multiples of pitch spacing at higher cost. If load spacing is important, it may be necessary to select a different type of chain.

All parts of these conveyors have loose fits and are normally not guided closely. Load carriers are seldom exactly alike and can hang at various angles and be out of line horizontally or vertically or both. This creates problems when attempting to operate limit switches and signal devices from conveyor parts.

Overload protection for conveyor drives must be provided when they are driven by electric motors. Those driven by pneumatic or hydraulic power can usually stall safely without damage. The most effective method utilizes a floating drive. The drive machinery is mounted on a platform that rests on wheels in a fixed frame. Chain pull is counteracted by springs so that floating frame position is a measure of force exerted. If chain pull exceeds designed value, a limit switch is operated to stop the conveyor. Drive frame can still travel farther to absorb energy of "drive parts without damage. This method is independent of speed.

Fixed drives can use an adjustable slip clutch with underspeed switch to indicate stall, overload cutout with parts to separate and operate a limit switch, or overcurrent relays. Signals from motor current are unreliable if a mechanical variable-speed device is used between motor and speed reducer.

Slack chain control must be provided since any chain will elongate from wear. If a rivetless chain is permitted to run too loose, pins may fall out or center links slip and lock crossways and cause jams on turns and drives. A takeup is usually a 1800 turn mounted movably so that effective track length can be changed. Movement is controlled by screws, screws with springs for manual adjustment periodically, or automatically by spring, counterweight, or adjustable pressure air cylinders. Travel of takeups is limited by the necessity of carrying chain and loads across a slip joint between the fixed and movable tracks, and providing sufficient strength in a limited space. Minimum travel must be sufficient to take out at least two pitches of chain and still permit chain coupling.

When equal load spacing must be maintained, provisions are made to move the whole takeup frame bodily each time the limit of travel is reached. It is usually necessary to cut the track and insert new sections to fill the gap. Eventually one complete space will be removed and the process started over again. A conveyor must be out of production while changes are made. Location of takeups is important, particularly for multiple drive conveyors. They are usually located at points of lowest tension or elevation. For example, a point past a drive exit between a dip and a rise would be ideal.

Automatic takeups must have sufficient power to keep a chain tight under any variable conditions of loading. This means that there may be several hundred to more than 1000 lb initial tension in a chain. Since each horizontal turn or vertical bend adds 2 to 10% to chain tension on entering side, excessively high chain loads can develop. Conveyors passing through ovens should have takeups located nearby so that when the heat is turned off and the chain contracts on cooling, it can be released to the oven with little force. Oven turns have been pulled down or damaged by lack of attention to this point.

Conditions of loading of a conveyor can affect selection of control elements. Most assembly conveyors are uniformly loaded. Those passing through units for chemical treatment, painting, etc., are frequently cleared each day and reloaded the next day. Storage types at times are heavily loaded in sections only. The problem is greatest when a conveyor passes through ovens and/or has many vertical bends and high lift loads. Chain pull lift load at the top of a vertical bend due to loads on the incline is equal to the live load per foot of conveyor times vertical height of lift. The difference between the lift load from empty carriers and that from loaded carriers is frequently much more than friction load for the entire conveyor.

Under some conditions there may be runaway forces tending to overspeed drives. If a conveyor must be stopped, drive brakes are required. Improper lubrication can double or triple normal drive pull requirements. This is important when variable-speed, constant-torque motors are specified. Most conveyors can be readily rearranged, shortened, lengthened, or combined. Model and method changes usually require conveyor rearrangements. Loads may increase in size, weight, and spacing. Controls and components should be selected for best adaptation to change as well as for standardization.

Adjustable speed requirements affect control means. Single drive conveyors usually use a variable-speed pulley or a variable-speed transmission either adjusted manually by handwheel on the drive or remotely by speed-changing motors. Conveyors with multiple drives or those which must run at precise speeds or in synchronization with other units require more elaborate controls.