Selecting the Proper Conveyor Belt
T1
Head Pulley
T2
Low Tension
+ Snub
Arc of Contact
Single Pulley Drive
T1
+
T1
Arc of Contact
Primary Pulley
T3
Arc of Contact
Secondary
Pulley
T2
Geared T andem Drive
Selecting the Proper Conveyor Belt
Selection of the best conveyor belt construction
. . .carcass . . . and cover . . . is based on the
Maximum Operating T ension
specific requirements of the particular conveyor Maximum operating tension is generally charac-
system, the material being conveyed and the terized in terms of pounds per inch of width and
conditions under which it must operate, as well should be matched as closely as possible to the
as its “cost effectiveness.” Some primary PIW working strength of the belt constructions
considerations involved are: under consideration.
1. Maximum Operating T ension (W orking Maximum operating tension is a prime consider-
T ension) ation in selecting the right belt . . . this being the
highest tension occurring in any portion of the
2. Minimum Pulley Diameters belt, on the conveyor system, under operating
conditions. The conveyor system applies an
3. T roughability , T ransverse Rigidity appropriate amount of power to the belt in order
to drive the belt at design speed. This power
4. Load Support must be sufficient to accelerateand drive the
empty conveyor, to move the material horizontal-
5. T ransition Distance ly and vertically , all within the design of the con-
veyor system . . . and to overcome all flexural,
6. Impact Rating inertial, frictional and gravitational forces operat-
ing on the system.
7. Covers, and
These aforementioned forces create tension in
8. Cost per unit handled the belt. The amount of tension created can be
computed in the time honored fashion by careful
Should this belt be a replacement belt, an consideration of each of these forces; however ,
examination of the old belt and a thorough study there is a “Quick Method” which can be used
of the conveyor system itself can pay off in and which generally proves satisfactory . Initially ,
longer belt life and reduced maintenance cost. let us consider effective tension.
Questions, such as the following, should be Effective tension (T e) is that tension created in
raised: the belt when sufficient power is applied to the
system to drive the conveyor belt at a desired
1. How did the old belt fail? speed. This relationship can be derived from a
knowledge of motor horsepower and belt speed
2. How long did it last? as follows:
3. What type carcass? Cover gauge? T e= Hp x 33,000
belt speed (in feet per minute)
4. Have operating or environmental conditions
changed? (Horsepower [Hp] usually refers to the power actually
applied to the belt. If, however , we simply use the
5. In view of the past history , what changes in nameplate horsepower rating of the motor in the system,
belting specifications are recommended? we automatically build in a convenient safety factor , pro-
viding the motor efficiency is less than 100%).
1
Belt conveyors utilize a friction drive and 3.T ype of take-up (whether screw or gravity).
accordingly, when power is applied to the drive
system, one run of the belt will experience a The ratio of slack side tension (T 2) and effective
higher tension than the other. Let us call this the tension (T e) can be represented by a constant.
tight side tension (T 1) and the other run, the slack
side tension (T 2). Upon installation, a belt is
normally tensioned until the belt fails to slip with K = T2 Therefore; T 2 = KT e
the system fully loaded. Te
The amount of slack side tension required to pre-
vent slippage at the drive is a function of several For convenience sake, a “K factor” table has
constant factors: been derived which takes these factors into con-
sideration. This table can be seen below.
1.The coefficient of friction between the drive
system and the belt (whether pulley is lagged Maximum operating tension (tight side tension)
or not). can now be computed by T 1 =T e + T 2 times the
starting factor (1.5, 2.0, 3.0 etc.). Electric motors
2.Belt wrap at the drive, and type of drive. T ype may have very high starting torques. CEMA rec-
of drive is important, since this has a direct ommends the use of a starting factor (multiplier)
bearing on how the motor applies driving to compensate in calculations.
force to the belt. This has a direct impact on
the maximum tension to which the belt will In the “quick method” we equate “maximum
be exposed. (See illustrations on the following operating tension” to “tight side tension” since
page). we are using a generous safety factor-total motor
horsepower .
K Factor T able
Screw Takeup Counter Weighted
or Gravity Take-up
Angle of Belt Wrap at Drive Type of Drive
Bare Lagged Bare Lagged
Pulley Pulley Pulley Pulley
180° Plain 1.20 .80 .84 .50
200° Snubbed 1.00 .70 .72 .42
210° Snubbed 1.00 .70 .67 .38
220° Snubbed .90 .60 .62 .35
240° Snubbed .80 .60 .54 .30
380° Tandem or Dual .50 .30 .23 .11
420° Tandem or Dual - - .18 .08
For belts exposed to the weather and wet operating conditions, the K factor in the calcula-
tions should be increased by up to 50% to help prevent slippage. By increasing the K factor,
the slack side tension (T 2 ) will be increased, thereby increasing the counterweight (Cwt).
