siegling

Establishing the lengthsof endless belts

belting

All lengths of spliced conveyor belts on orders and spareparts cards refer to what’s known as the inside length (Li)when the belt has not been tensioned. This dimension is
not the same as the length of the neutral axis. Only Li isused as the length for orders. (See fig. 1 for an illustrationof these two figures)
The inner length (Li) is either established based on the con­veyor’s dimensions or a belt sample. When measuring a belt
sample, the inner lengths of both belt edges are established.
If not otherwise agreed with Forbo Movement Systems, Li is
calculated as a mean of these two measurements. Dependingon the size of the conveyor and belt, various methods can beused to establish the inner length (Li):

a0Lnf

nf

Li

a0 = belt thickness		
nf = height of the neutral axis
LnF = length of the neutral axis = flat length (red)		 The layer that isn’t subjected to elongation or compression
Li = inner length of the belt (green)
The following applies:
Inner length (Li) + thickness allowance = length of the neutral axis (LnF)
In dual-pulley drives with a 360° arc of contact, the thickness allowanceis calculated as follows: Thickness allowance = 2 x nf x π
The height of the neutral axis (nf ) can usually be assumed as half of thebelt thickness a0. Or, in more simple terms: Thickness allowance= a0 x πWith respect to general tolerances, the thickness allowance for belt
lengths as of 3 m is virtually immaterial. However, in the case of shorter
belts and tighter length tolerances, it does have to be taken into accountwhen checking measurements.

1 Carrying out measurements on the conveyor
Insert a thin and accurate tape measure into the path of the belt
and measure the spliced inner length (Li). In order to ensure
there are no problems with fitting the belt later on, do not fully
retract tension drums if there are any. The allowance for belt
elongation should be at least 0.2% of the belt length. In a CAD
layout, Li is the length of the thin line that is placed tangentiallyaround the rollers and drums. As the tape measure (or thedrawn line), for example on an Ω drive, is sometimes on the top
face or the underside of rollers, there will be a small but accepta­
ble measurement error. (Fig. 2)

Fig. 1
Measurement path

Fig. 2
Fig. 3

2 	Measuring the inner length directly(for thicker, relatively narrow belts)
If the belt length is easy to handle, the inner length (Li) can bemeasured with a tape measure placed on the inside. (Fig. 3)
When smaller belt sizes are produced in series, it might beworthwhile setting up a measurement device with two returns,
one of which can be moved. A thin tape measure must be used
to calibrate the measurement device beforehand. In order tomeasure it, tension the belt with very little force, the level ofwhich might have to be agreed with Forbo Movement Systems.
In order to pretension the belt, the weight of a return pulley canbe used if the device is mounted vertically. (Fig. 4)

Fmin

Fig. 4

Establishing the lengthsof endless belts

3	Calculating the inner length (Li) from the flat length afterpressing the belt flat(for thin, single-ply belts up to approx. 2 mm thick)Press the belt flat (prevent any undulations from forming) andmeasure from the inside edge of one fold to the inside edge ofthe second fold.

If you use this method, please discuss with
Forbo Movement Systems beforehand.
Caution: Never use this method for belts witharamid or polyamide tension members.

As a small radius always remains on the folds, deduct 2 mm.
0Flat length = (2 x length measured) + 2 mm
(in the example: (2 x 528 mm) + 2 mm = 1058 mm

Fig. 54 	Measuring the flat length by moving
the belt and calculating the inner length (Li)(where larger belt sizes apply)
In this case, the belt is rotated and measured in sections andeach of the sections are marked exactly on the belt. Adding
each of the measured sections produces the flat length(= length of the neutral axis). (Fig. 6)
In the case of narrow belts, it’s often sufficient to apply just one
marking to the belt and to rotate the belt once next to a tapemeasure. The flat length can then be established directly. Inorder to prevent the belt from slipping and undulations fromforming in the material, the belt must be pressed really flat onto
the surface that it touches. (Fig. 7)
In an flat state, belts with mechanical fasteners are very easy tomeasure (from the middle of one eyelet to the middleof the next). (Fig. 8)
In all three cases, the inner length (Li) is then calculated asfollows:

In this case, themeasurement of 1 m
is just an example
1m1m

1m0Fig. 60Fig. 70Fig. 8

Inner length (Li) = flat length – thickness allowance** Thickness allowance see explanation of fig. 1Forbo Siegling GmbHLilienthalstrasse 6/8, D-30179 HannoverPhone +49 511 6704 0
www.forbo-siegling.com, siegling@forbo.com
Forbo Movement Systems is part of the Forbo Group,a global leader in flooring and movement systems.
www.forbo.com

10/23 · Reproduction of text or parts thereof only with our approval. Subject to change.

* Thickness allowance see explanation of fig. 1

47 48 49 50 51 52 5 30Ref. no. 591-2

Inner length (Li) = flat length – thickness allowance*