10. Chain Pull (P)

In calculating the chain pull or force for the chain, ancilliaries and load to be conveyed we must refer to the following factors.

10.1 Chain Loading including material defined as M, chain weight and ancililaries defined as C. See consideration 4.
10.2 Co efficiency of friction. This is defined as the force required to overcome the resistance to movement between two surfaces. Chain within conveyors must overcome sliding friction defined as follows.

F1 Coefficient of friction where chain sliding or rolling on a wearstrip.

F2 Coefficient of friction where material is sliding within a trough.

Chain Friction factors F1 – Chain Sliding.

Chain sliding on steel track – unlubricated 0.35
Chain sliding on steel track – lubricated 0.20
Chain sliding on rough track – unlubricated 0.45
Chain sliding on rough track – lubricated 0.25
Chain sliding on non metalic wearstrip 0.18 (typically UHMWP)

It is important to consider that at start up the coefficiency of friction is 1.5-3.0 times greater than the dynamic friction coefficient. As a general guide, in order to minimise start up friction the roller diameter should be a minimum of 2.5 times greater than bush diameter.

When running on rollers; chain must overcome both sliding and rolling Friction F3.

The value of the rolling coefficient in the initial calculation is assumed as F3 = 0.2, whilst in the control calculation is given as

    \[ F3=C\frac{d}{D}+\frac{b}{D} \]

where d=bush O/D (mm), D=Roller O/D (mm), b=coefficient dependant on material employed and quality of contact surfaces. This is equal to 1 for stell roller on a steel track with smooth surface and 2 for a steel roller on steel track with rough surface. The sliding friction coefficient defined as C between bush and roller is as table 10.2.

Table 10.2

Bodies in contact

Dry surface “C”

Lubricated surface “C”

Steel roller on steel busz

0.25

0.15

Roller with bronze bush on steel bush

N/A

0.13

Nylon roller on steel bush

0.15

0.10


Chain Friction Factors F1 chain rolling.

    \[ F1=fr\frac{da}{dr} \]

where da=axle diameter (usually chain bush O.D and Dr=Roller outside diameter)


10.3 Service factor – SF

Chain pull must include a multiplier to take into account the variable operating conditions of the conveyor. This is known as the service factor. Variations in operating conditions and the service factor (SF) as defined in the following table 10.3.

To obtain the SF each operational condition must be multiplied together.

Table 10.3 Operating conditions

FS

Load position

- Central

1

- Not Central

1.2

Load characteristics

- Uniform: extent of overloading less than 5%

1

- With minor variations: extent of overloading 5 to 20%

1.2

- With major variations: extent of overloading 20 to 40%

1.5

Frequency of loaded starting/stopping

- Less than 5 per day

1

- From 5 per day to 2 per hour

1.2

- More than 2 per hour

1.5

Working environment

- Relatively clean

1

- Quite dusty or dirty

1.2

- Humid, very dirty or corrosive

1.5

Number of hours in use daily

- Up to 10

1

- More than 10

1.2



To obtain the total S coefficient, (FS) value for each operational condition must be multiplied together.

Gearing Factor FA

This is an adjustment coefficient made to the chain pull, which increases due to the additional friction caused by the rotation of the chain on the drive and driven wheels.

FA = 1.05 for wheels mounted on brass bushes
     = 1.03 for wheels mounted on bearings

The sum of all products obtained by the multiplying FA for the chain pull in each gearing point determines the new total chain pull.
For the following examples the “FA” values will not be considered.