V-BELT DRIVES WITH TWIST AND NON-ALIGNMENT
V-belts can be used advantageously to transmit power between driving and driven sheaves that are misaligned with
respect to each other, within prescribed limits. The belts on such drives will operate under conditions of offset and/or
twist. Offset causes the belt to enter and/or leave the sheave obliquely and to rub one side of the sheave groove
more than the other. 1/8 and 1/4 turn drives put both twist and offset into each span of the belt. Such drives will give
less service life than those where the sheaves are aligned. Offset should be as small as possible, and in all cases it
should be limited to an offset angle (see dimension “g” in offset drive figure) into or out of the sheave groove no
greater than 5 degrees. Excessive offset results in abnormal wear and/or belts turning over or jumping out of the
sheave grooves. Banded V-belts may be used to minimize stability problems. High modulus belts are not recommended because of their intolerance to sheave misalignment. Inherently these particular drives create extreme
stresses on the reinforcing tensile members. The high modulus of elasticity of the tensile members prevents the
absorption of the stress imposed without damage. Deep groove sheaves are recommended for V-belt drives where
belt offset exists, but standard grooves must be used for banded belts.
Design Recommendation A
Mule Drives and Other Drives Using Twisted Single Belts
OFFSET DRIVE
MULE DRIVE
g
OFFSET
1/4TH TURN DRIVE
90˚
1/8TH TURN DRIVE
45˚
Minimum Belt Spans Between Sheaves at 90˚ Twist of the Belt
Belt Cross
Section
Span
(inches)
Belt Cross
Section
Span
(inches)
Belt Cross
Section
Span
(inches)
A
B
C
D
9
11
14
19
3V
5V
8V
9
14
19
3L
4L
5L
4
8
10
Cogged or notched V-belts may be used in these applications but are generally not recommended.
Minimum span for other twist angles (”A” degrees) is Span at 90˚ x A
90
Example: Minimum span for a C section belt with 1/8 twist or turn (45˚) is 14″ x 45 = 7″
90
V-BELT DRIVES WITH TWIST AND NON-ALIGNMENT
(Continued)
Design Recommendation B
Multiple V-Belt Quarter Turn Drives
90˚
TOP VIEW
VERTICAL SHAFT
HORIZONTAL SHAFT
SIDE VEIW
5.5 (D+Fb ) MIN.
D
e
Fb
TIGHT SIDE
VERTICAL SHAFT
HORIZONTAL SHAFT
These have both twist and offset. (Use deep groove sheaves.)
Minimum Center Distance
The drive should have a minimum center distance of 5.5 (D + Fb),
Where:
D = diameter of the large sheave
Fb = width of the band of belts
The width of the band of belts is given by the following formula:
Fb = (Nb - 1)Sg + bb
Where:
Nb = number of belts
Sg = deep groove spacing
bb = nominal belt top width
Note: This will ensure that the angle (g) of the belt entry of exit does not exceed 5 degrees.
V-BELT DRIVES WITH TWIST AND NON-ALIGNMENT
(Continued)
Location of Sheave on Vertical Shaft of Horizontal Quarter Turn Drives
The center of the face of the sheave on the vertical shaft should be below the axis of the horizontal shaft by the
amount “c” which depends on the center distance.
Offset Dimension “e”
Center Distance
(inches)
Dimension “e” (inches)
A, B, C, D
Cross Sections
Dimension “e” (inches)
3V, 5V, 8V
Cross Sections
20
30
40
50
60
80
100
120
140
160
180
200
220
240
0.2
0.2
0.4
0.4
0.5
0.5
1.0
1.5
2.0
2.5
3.5
4.0
5.0
6.0
0
0
0
0
0.2
0.3
0.4
0.6
0.9
1.2
1.5
1.8
2.2
2.6
Arc Correction
The arc correction factor for a quarter turn drive should be taken as 1.0.
Direction of Rotation
Direction of rotation must be such that the tight side of the drive is on the bottom.
Location of Vertical Shaft
The axis of the vertical shaft should lie in a plane perpendicular to the horizontal shaft, and intersecting it at the
center of the face of the sheave on the horizontal shaft.
Larger Speed Ratios
For speed ratios greater than 2 1⁄2 : 1, the shortest center distance permissible may be too long for the usual quarter turn drive. It is suggested that an arrangement such as illustrated in the figure below be used. This consists of
a quarter turn drive between the higher speed shaft, a jackshaft and a regular multiple V-belt drive (V-V or V-flat)
between the jackshaft and the slower speed shaft.
TOP VIEW
HORIZONTAL
SHAFT
VERTICAL JACK
SHAFT