2015 Designer's Notebook - 142

Sofstep® Selection
Material

Tension
(lbs/inch of web width)

Aluminum foils
Cellophanes
Acetate

0.5 to 1.5 (1.0 avg)/mil
0.5 to 1.0/mil
0.5 to 1.0/mil

Mylar (Polyester)
Polyethylene
Polypropylene
Polystyrene

0.5 to 1.0 (.75 avg)/mil
0.25 to 0.30/mil
0.25 to 0.30/mil
1.0/mil
0.05 to 0.20 (0.10 avg)/mil
0.05 to 0.20 (0.10 avg)/mil

Vinyl

Paper and Laminations
20#/R-32.54
gm/m2
40#/R-65.08
gm/m2

0.50 to 1.0
1.0 to 2.0

Calculating Web Tension
For sizing brakes on applications in which
the applied web tension is unknown, use
the following information to determine the
approximate tension value.

Applied Web Tension =
Approx. Material Tension x Roll Width

Example:
The tension for a twelve inch wide roll of
20# paper stock is unknown. What is the
prescribed tension?

Paper
0.5

Solution:

20 lbs/ream
30 lbs/ream

0.75
1.0

The approximate tension value as noted in
the chart to the left for 20# paper stock
is 0.75 lb/in; thus the tension for this
application is (0.75 lb/in x 12) = 9 lbs

Laminations
25 lb paper/.005 inch
PE/.00035 inch
Foil/.001 inch PDE
.001 inch Cello/.0005 inch
PE/.001 inch Cello

3.0

1.5

Magnetic Particle Clutches & Brakes

When these substrates are coated with polyethylene, nylon,
polypropylene, EVA, EAA, and EEA, add the following tension
to the values listed above for the substrate only.
Coating Thickness
0.00005 inch to 0.001
0.0011 inch to 0.002

0.12
0.25

Cellophane
.00075 inch
.001 inch
.002 inch

0.5
0.75
1.0

Nylon and Cast Propylene (Non-Oriented)
.00075 inch
.001 inch
.002 inch

0.15
0.25
0.5

Paperboard
8 pt.
12 pt.
15 pt.

3.0
4.0
5.0

Inertia (WK2)
The inertia of the components must
be known or calculated for cycling
applications and for controlled starting
or stopping applications.
For a simple rotating cylinder:

Inertia = WK2
Where:

W = Weight of body
K = Radius of gyration
The equations for calculating the radius of
gyration of a cylinder about its axis are
given in the illustration below:

Mylar and Oriented Propylene
.00005 inch
.001 inch

0.25
0.5

.002 inch

1.0

Material

lbs/strand
Aluminum Wire
4.00
5.50
9.00

#14 AWG

The formula for determining the equivalent
WK2 of a rotating part referred to the
clutch or brake shaft is:
2
WK e2 = WK2 x ( N )
Ncb

Where:

15 lbs/ream
(3000 sq. ft.)

#20 AWG
#18 AWG
#16 AWG

pulleys, etc., do not necessarily operate at
the same speed. In clutch and brake
problems it is common practice to
calculate the WK2 of the parts operating
at each speed and reduce them to an
equivalent WK2 at the clutch or brake
mounting shaft speed, so that they can all
be added together and treated as a unit.

10.00

Reflected Inertia (Rotational)

Copper Wire
#20 AWG
#18 AWG
#16 AWG

8.00
10.00
12.00

#14 AWG

15.00

142 Magnetic Particle Clutches & Brakes

In most practical mechanical systems the
rotating parts such as gears, drums, rolls,

WK2 = inertia of the rotating part at N (RPM)
N = speed (RPM) of the rotating part
Ncb = speed (RPM) of the clutch or brake shaft

Reflected Inertia (Linear)
There are also complex systems involving
both linear and rotating motion. The
inertia of the linearly moving parts can
also be reduced to the clutch or brake
speed by the equation:
2
WK 2e = W x ( V )
2πN

Where:
W = weight of the body
V = velocity in feet per minute
N = RPM of the clutch or brake shaft

This equation can be used only where
the linear speed bears a continuous fixed
relation to the rotating speed, such as a
conveyor driven by a motor, etc.
Thus it can be seen that it is possible to
reduce the WK2 of the individual parts of a
complex system to an equivalent WK2 at
the clutch or brake shaft speed. These
values of equivalent WK2 may be added
directly and the total equivalent WK2 plus
the WK2 of the clutch and/or brake parts
represent the total WK2 of the complete
system which the clutch or brake must
accelerate or decelerate. From the above
formula it is apparent that parts
operating at speeds substantially lower
than the clutch or brake mounting shaft
speed are usually a small factor in the
total equivalent WK2.



Table of Contents for the Digital Edition of 2015 Designer's Notebook

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2015 Designer's Notebook - Cover3
2015 Designer's Notebook - Cover4
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