Roller Table Conveyor

Roller Table Conveyor

Roller conveyor systems are used as mechanical handling equipment which moves materials from one location to another. These systems provide quick and efficient transportation for a wide variety of materials, which make them very popular for the material handling in the industries.

Roller conveyors are essentially used to coveys unit loads with at least one rigid, near flat surface to touch and maintain stable equilibrium on the rollers, like ingots, slabs, blooms, billets, plates, rolled stock, pipes, logs, boxes, crates, and moulding boxes etc. A roller conveyor supports unit type of load on a series of rollers, mounted on bearings, resting at fixed spacings on two side frames which are fixed to stands or trestles placed on floor at certain intervals. The spacing of rollers depend on the size of the unit loads to be carried, such that the load is carried at least by two rollers at any point of time.

Roller conveyors are classified into two groups according to the principle of conveying action. These are namely (i) unpowered or idle roller conveyor, and (ii) powered or live roller conveyor.

In an unpowered roller conveyor, the rollers are not driven or powered from an external source. The loads roll over the series of rollers either by manual push or push from an endless moving chain or rope fitted with pusher dogs, rods or clamps. The unpowered roller conveyors normally operate in horizontal plane, but at times a gentle slope is given to these conveyors to aid motion of the loads. An inclination of 1.5 % to 3 % ensures that the load rolls by gravity. Such conveyors are termed ‘gravity roller conveyor’.

In case of a powered roller conveyor, all or a selected number of rollers are driven by one or a number of motors depending on the selected drive arrangement. The driven rollers transmit motion to the loads by friction. The powered roller conveyors can be installed at a slightly inclined position, upto 10 degrees up or upto 17 degrees down. The load can be moved in both the directions by changing the direction of rotation of the rollers, where these are called reversing roller conveyors.

Roller conveyors are used for conveying almost any unit load with rigid riding surface which can move on two or more rollers. These are particularly used between machines /equipments, in rolling mills, warehousing, docks, foundries, manufacturing, and in assembly and packaging industry. These conveyors are also used for storage between work stations and as segment of composite handling system.

However, the limitations of rollers conveyors are that they can be best used for the objects having rigid flat surfaces, and for movement to relatively short distances. These conveyors need side guards to retain the loads from falling off. Gravity roller conveyors also have the risk of accelerating the loads.

Unpowered roller conveyors

The unpowered roller conveyors consist of series of rollers, the frame on which the rollers are placed, and the stands also called the trestles on which the framework rests. Because of simplicity of design, competitive cost, and trouble free operation, these conveyors are used extensively in handling unit loads in workshops or process plants to convey articles from one working station to another.

Unpowered roller conveyors are frequently used as a storing platform and as such are frequently termed as roller table. These are also used in stores as storing racks and in loading bays for loading / unloading materials from carriages. A gentle slope can be provided in the conveyor to aid movement of the loads on idle rollers. These gravity roller conveyors are used to convey load in one direction only.

The conveyors can have a curved section to change direction. Material movement between two levels can be done by an inclined or a spirally formed gravity roller conveyor. The spiral form increases the length of the conveyor and thereby controls the velocity of the articles moving down the conveyor.

Parts of unpowered roller conveyors

The different parts of unpowered roller conveyor are described below.

Rollers – Cylindrical rollers are normally used which are made from ERW (electric resistance welded) steel pipes with cast or fabricated end flanges to accommodate the anti-friction bearings (normally ball bearings). The through axles are stationary and roller barrels can rotate freely. These rollers are called idler rollers. For conveying, cylindrical objects (drums, pipes, round steel bars etc.), double tapered rollers, or wheel rollers are used. Fig 1 shows different types of rollers.

Fig 1 Types of unpowered conveyor rollers

The diameter of the rollers depend on the diameter of standard steel pipes available, and varies from around 20 mm to a maximum of 150 mm. Heavier the load to be conveyed, larger diameter and heavier wall thickness of the rollers are needed. Typical sizes of some of the rollers and their weight carrying capacities are given in Tab 1.

Tab 1 Typical roller sizes and weight carrying capacities
Sl. No. ParameterUnitType of roller
MediumHeavyExtra heavy
1Roller diametermm75100150
2Maximum load per rollerkg3006001,200
3Axle diameter at the journalmm203045

Roller pitch depends on the length and weight of the load handled. The unit load is required to be supported at least by two rollers, thus the maximum pitch is to be less than or equal to 0.5 times of the length of the load. For goods vulnerable to jerks / shaking, roller pitch equal to 0.2 times to 0.25 times of the length of load is to be considered for designing the roller conveyor.

Frame – Frame is that part of the conveyor on which the roller axles rest and are fixed to. The conveyor frame is fabricated from angle or channel sections. The roller axles are held in slots cut in the flanges of the frame. The axles are flat machined at the ends so that the axles do not rotate in the slots. Axial movement of the axles is prevented by using split pins or lock plates. For heavy rollers, the axles can be fixed on the frame by clamps. Typical idle rollers with bearing fittings and their attachment to the frame are shown in Fig 1. Side guards can be provided along two edges of the frame to prevent movement of the loads beyond the roller span. Side guards are particularly necessary at the curved sections of a conveyor.

Stands or trestles – Stands or trestles support the conveyor frames with roller assemblies, from the ground. Stands are normally fabricated from pipes or structural sections, with provision for grouting on the floor. Height of stands is chosen to keep the articles at a convenient level on the conveyor. Small portable conveyors frequently have telescoping legs for the stands, such that the inclination of the conveyors can be suitably adjusted in situ.

Special designs / features of unpowered roller conveyors

A number of features can be incorporated in an unpowered roller conveyor to satisfy different functional requirements. Some of these are described below.

Double-row roller conveyor – This design is used to convey wide and heavy loads. In place of one long and proportionately large diameter roller, two smaller diameter rollers with lengths less than half of the larger roller are used. For the support the inner ends of the pair of rollers, additional support frame is used. This design has lower cost than the same width conventional single row conveyor with longer and larger rollers.

Curved sections – These are used for changing direction of the conveyor in the horizontal plane. In this section rollers are arranged radially. Two or more numbers of cylindrical rollers are used in place of one roller to reduce sliding action of the load on the rollers. Minimum sliding action of the load on rollers can be achieved by using single tapered roller (V-roller), but as these are to be made from solid casting or forged section, are costly and rarely used.

Switches – Devices used in a roller conveyor to change the normal direction of the load or divert the load from the conveyor are called switches (Fig 2). Various types of switches are used such as (i) a turn table, (ii) a hinged section, (iii) swiveling head, and (iv) defector etc.

Fig 2 Switches on unpowered roller conveyor

A turn-table (Fig 2) is used for transferring a load from one roller conveyor to a sliding runway or to a perpendicular roller conveyor. This consists of a small length (equal or slightly smaller than the width of the main conveyor) of roller conveyor mounted on a base frame which is fixed on a vertical shaft and mounted on bearings. Once the load comes on the turn-table, the turn table can be rotated (manually or by proper mechanism) to the desired angle and the load can be rolled over to the desired runway.

A hinged section (Fig 2) is a small section of the conveyor which is hinged at one end with the frame / stand of the main conveyor, and can be lifted up to make a passage way through the conveyor line. An arrangement of wheel rollers on swiveling heads (castor wheel) are used independently or as a part of a roller conveyor where it is necessary to move the load in many directions.

Deflector is a flat or angle like section placed longitudinally over the conveyor making an angle (can be adjustable or fixed) with the conveyor axis. This acts as an obstruction to the movement of the load and deflects them to one side of the conveyor axis. Manipulator consists of one pair of deflectors to bring the loads at the middle section of the conveyor axis. Deflector or manipulator is used on idle conveyors and more frequently with powered roller conveyors with chain / rope pushing facility.

Stops – These are placed at the end of the conveyor to physically stop the moving loads from falling off the conveyor end. Disappearing stops can be placed at desired intermediate points in the path of a roller conveyor to stop the moving articles at such points, if needed. The stops are simple flat steel plates fixed on rigid legs or fixed to conveyor structure. Disappearing stops can be moved up or down from the top level of the roller by suitable mechanism.

Powered roller conveyor

Powered roller conveyor is also called ‘live roller conveyor’. In powered roller conveyor, all or a few of the rollers are driven by one or multiple motors through associated transmission system. The loads on the roller conveyor are moved by the frictional force caused between the loads and the driven rollers supporting the loads. Powered roller conveyors are intensively used in heavy process plants like rolling mills to feed heavy and at times hot rolling stocks to feed the rolling mill or to take delivery from the mill and send to various other process equipments. The roller conveyors can be reversing type to suit the process or can be non-reversing type which transports materials only in forward direction.

Parts of powered roller conveyors

The different parts of powered roller conveyor are described below and shown in Fig 3.

Fig 3 Parts of driven rollers

Rollers –  The rollers of a powered roller conveyor (Fig 4) is fundamentally different from those of the unpowered roller conveyor in that the barrel and the shaft portion are integral so that they can be driven by connecting power to their shaft ends. The integral shafts are mounted on bearings housed in the frames at two sides. These are termed as driven rollers.

The driven rollers are normally subjected to considerable impact load (specially the reversing type processing conveyors) and hence they are made stronger. The rollers can be made from solid steel forgings or castings or can be fabricated from heavy section of pipes, tubes, and solid shafts, machined all over for proper static and dynamic balancing. The diameters can be varying between 400 mm to 600 mm for roller tables used in heavy slab, or blooming and billet mills, down to 250 mm to 350 mm for general duty transporting conveyors.

Roller pitch is so selected that the load is supported by at least two driven rollers. To prevent sagging of the load between two driven rollers, non powered (idle) rollers can be introduced between two driven rollers.

Fig 4 Rollers for powered roller conveyors

Frames – The rollers are supported at their journals on two set of frames at two ends. The frames are connected by heavy tie rods to make a composite frame structure suitable for grouting the conveyor frame on its foundation. For a heavy duty conveyor, the framework is normally made from cast steel, and for a lighter duty conveyor, the frames can be fabricated from rolled steel plates and sections. Design of the frames mainly depends on the drive system used for the roller conveyor.

Drive arrangement – Main classification of powered roller conveyor is based on the type of drive arrangement used. When one motor drives more than one or all the driven rollers, it is called ‘group or multiple-drive’. In group drive, normally only one motor with suitable transmission arrangement is used to drive all the driven rollers. For a long conveyor, or from other considerations, more than one motor can be used, each driving a group of rollers in different sections of the conveyor. The transmission of power from the motor to the rollers varies widely depending on the use.

When each of the driven rollers is driven by an individual motor, it is called individual drive. These motors can be high speed motors transmitting motion through a reducing gear (Fig 4). Alternatively, specially designed slow speed hollow rotor shaft motors are used which are directly coupled to the roller shaft. With the availability of better electrical control systems, individually driven roller conveyors are getting more popular particularly for reversing duty.

In a heavy duty non-reversing conveyor, bevel gear transmission arrangement can be used. The motor, through a gear box drives a shaft placed along the length of the drive side of the conveyor. Power to all the rollers are through set of two bevel gears as shown in Fig 4. The drive shaft with supporting bearings and the bevel gears are housed in the box frame, and partially immersed in oil for lubrication. In an alternative design the transmission of power can come to one roller, and the other driven rollers can be connected to this driven roller by series of sprockets and chains.

In a light duty powered roller conveyor, the rollers can be driven by one endless flat belt driven below the rollers, and supported by idle rollers such that the belt touches all the rollers and transmit power to them by friction. This is, unlike others, not a positive drive.

Portable roller conveyor

Portable roller conveyor is a short (upto 7 m) section of roller conveyor mounted on legs and at times with wheels. This conveyor can be shifted from one place to another and adjusted in height or inclination for loading and unloading of trucks. The portable roller conveyor can be either idle or driven. Drive conveyor is frequently through an endless belt described under power roller conveyor.

Design aspects of roller conveyors

The design aspects of the roller conveyors are described below.

Unpowered roller conveyors – The major design calculations involved are to determine the force needed to overcome the resistance to motion of the loads and the angle of inclination needed for a gravity conveyor. Total resistance to motion is made up of (i) resistance to rolling of the load on rollers due to friction, (ii) frictional resistance in the roller bearings, and (iii) resistance due to sliding of the load on the rollers and force needed for imparting kinetic energy to rollers.

Resistance ‘F1’ to rolling of the total load ‘G’ on the rollers is given by the equation F1 = G x (k/R) where ‘k’ is the rolling friction factor also called coefficient of rolling resistance is given in centimeters (cm), and ‘R’ is the roller radius in cm.

Frictional resistance ‘F2’ on roller journals is expressed by the equation F2 = (G + w x n) x (M x r/R), where, ‘w’ is the weight of rotating part of each roller, ‘n’ is the number of rollers supporting total load, and hence in motion, ‘M’ is the coefficient of friction at the journal, and ‘r’ is the radius of the journal.

When a moving load comes over a static roller, it slides over the roller and starts accelerating the roller, till the roller attains the surface speed equal to speed of the load. When the load leaves the roller, it starts decelerating and eventually stops until it is accelerated by the next load. This phenomenon is shown in Fig 5.

In Fig 5, ‘O’ is the point of time when a moving load touches a static roller. Till time‘t’, the load rolls and slides over the roller. The surface or rotational velocity of the roller is under acceleration and is expressed by line ‘OA’. From point ‘A’ upto point ‘B’, the load rolls over the roller, and at ‘B’ it leaves the roller.

If the conveyor is conveying Z pieces of load per hour, then the cycle period is t1 which is 3600/Z seconds. If G′ is part of weight of each load carried by the each roller and Mo the kinetic coefficient of friction, the frictional sliding force between the load and roller during time t′ is equal to G′ x Mo and the work done by the load is equal to G′ x Mo x v x t′ where v linear velocity of the load, and v x t′ is the distance moved by the load in time t′ , represented by the area OEAF in Fig 5.

Fig 5 Velocity diagram of a roller

The distance travelled by any point on the periphery of the roller during this time is v/2 x t′ (area OAF), which is also the sliding path. This shows that half of the work done by load is spent in overcoming the friction, and the other half is used in imparting kinetic energy to the roller. If ‘w’ is the weight of the rotating part of the roller, then its kinetic energy is equal to 1/2 (w/g) x v2 x q. Where q is a factor having value between 0.8 and 0.9, because not all the mass of the roller moving parts is on the periphery, and hence not moving with velocity v. Hence, the work done due to sliding and acceleration of one roller is given by 2 × (1/2 w/g) x v2 x q = (w x v2)/g x q.

If there is ‘n’ number of rollers in a total length of ‘L’, then the total work done in ‘n’ number of rollers is (n x w x v2 x q)/g for moving one load throughout the length of the conveyor. Hence the average resistance to motion on one load due to sliding and acceleration is F3′ = (n x w x v3 x q)/(g x L). If there are Zo numbers of loads moving simultaneously on the conveyor, then average total resistance due to sliding and acceleration is F3 = (Zo x n x w x v2 x q)/(g x L). Hence, total resistance to motion of the loads, which is the force needed to move the loads on a horizontal unpowered conveyor is F = F1 + F2 + F3 = G x (k/R) + (G + w x n’) x [(M x r)/R] + q x [(Zo x n x w x v2)/(g x L)].

The equivalent resistance can be defined to motion factor ‘f’ by an equation, F = G x f. The value of ‘f’ is given by equation f = F/g =2k/D + [1 + (w x n’)/G] x (M x d)/D + q [(Zo x n x w x v2)/(g x L x G). Here ‘D’ is the roller diameter which is 2R, and ‘d’ is the journal diameter which is 2r.

However, for calculating the minimum inclination angle ‘X’ of a gravity conveyor, which allows movement of a load due to gravity only, resistance to only one load need to be considered, which is to be overcome by the component of the gravitational force on the load along the inclination of the conveyor. Thus, f = tan X = F/G’ = 2k/D + [1 = (W x n”/G’) x (M x d)/D] + q x (n x w x v2)/(g x L x G’), where, n′′ is the number rollers supporting each load and is equal to n’/Zo, G’ is the weight of each load and is equal to G/Zo. Hence n”/G’ = (n’/Zo)/(G/Zo) =n’/G. These equations essentially means, that in a gravity roller conveyor, with requisite inclination angle ‘X’, one or many loads placed on the conveyor move unaided due to gravity.

Powered roller conveyors

Power roller conveyors are the transport conveyors. The rollers of a group driven transport conveyor are rotated continuously in one direction irrespective of loads being on the conveyor or not. If the conveying rate is ‘Q’ tons/hour, ‘V’ and ‘Lh’ are vertical and horizontal components of the length ‘L’ of the conveyor in meters, ‘n’ is the number of rollers, ‘w’ is the weight in kg of rotating parts of each roller and the conveying speed is ‘v’ meters/second, then the motor power ‘P’ in kW is the sum total of power requirements for (i) raising the load through the vertical distance, (ii) rolling the load on the rollers, and (iii) rotating the rollers against journal resistance.

Thus, P = [(Q x V)/367 + (Q x Lh)/367 x {2K/D + (M x d)/D} + (n x w x v)/102 x (M x d)/D] x 1/e in kW. Here ‘e’ is the efficiency of the drive system. For a horizontal conveyor, V = 0, and Lh = L, hence P = [(Q x L)/(367D) x {2k + M x d} + (n x w x v x M x d)/102D] (1/e) in kW. If the weight G′ of each load in kilogram (kg) and number of pieces ‘Z’ transported per hour is given, then the above equation becomes P = [(G’ x Z x L)/36700D x (2K + M x d) + (n x w x v x M x d)/102D](1/e) in kW.

If the unit loads are fed uniformly, then the interval between two loads is given by t = 3600/Z seconds and time through which each load moves in conveyor is given by T = L/v seconds. Hence the number of loads moving simultaneously on the conveyor is Zo = T/t = (Z x L)/3600v pieces. Hence, the needed motor power for a horizontal conveyor is P =  [{(Zo x G’ X v)/102} x {2k/D + (M x d)/D} + (n x w x M x d x v)/102D] x (1/e) in kW or P = [(Zo x G’) x (2k + M x d) + (n x w x M x d x v)]/(102D x e) in kW.

Reversing conveyor

In a reversing processing conveyor, the direction of rotation of the driven rollers is changed frequently. As a result, the additional inertial forces for accelerating or decelerating the rollers and the load have to be taken into consideration. The maximum peripheral acceleration, ‘a’ of the roller is kept within limits, such that the load weighing ‘G’ moves on rollers without sliding (also called skidding). There is no sliding when the frictional force between roller and load is more than the inertial force needed to accelerate the load. If ‘Mo’ is the static coefficient of friction then, G x Mo is greater than or equal to (G/g) x a so that ‘a max’ is equal to Mo x g and the corresponding maximum angular acceleration ‘ar’ max of roller is given by equation ar max  = a max/r = 2a max/D = (2Mo x g)/D.

When a driven roller is accelerated, the load it carries is also accelerated, and this load can be considered as a rotating mass at the periphery of the roller being accelerated. This mass is G/g. But, the acceleration of the motor am = i x ar, where i = transmission ratio. Hence the inertial torque on the motor shaft Tim = [(Ir x n) + (G/g) x (D2/4)] x [ar/(i x n)] + (Im x am), where ‘Ir’ is the moment of inertia of each roller, and ‘Im’ is the moment of inertia of rotational part of motor. The total torque needed at the motor end is sum of inertial torque and static torque needed for overcoming the journal friction at rollers and the torque needed for rolling of the load over rollers.

Static torque at roller end is given by the equation Tsr = [(G + n x w) x M x d/2] + G x k. Static torque at motor end ‘Tsm’ is given by the equation Tsm = Tsr/ (I x e). So total torque ‘Tm’ at motor end is given by equation Tm = Tsm + Tim = [{(G + n x w) x (M x d)/2} + (G x k) + (Ir x n x ar) + {(G x D2)/4g) x ar}] x (I/i x e) + (Im x am).

The maximum torque of a reversing conveyor drive motor is so chosen that the motor does not stall even if the load does not move, i.e. the rollers can skid under the static load. The skidding torque ‘Tskid’ is given by equation Tskid = [{(G + n x w) x (M x d)/2)} + (G x Mst x D/2)] x (I/(i x e).

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