Given the following
requirements, select an acme screw for an application which uses
Acme screws for an automatic part feeder on a machine.
SPECIFICATIONS:
- 5000 lb load supported and guided on linear
bearings moving horizontally
- 36" travel
- Complete 36" travel in 10 seconds
- Bearing Support Undecided
- Positioning accuracy ±1/4"
STEP
1 Find the axial force required to move load.
The axial force is determined by multiplying
the coefficient of friction of the guidance system by the load.
F = µ x N
µ = coefficient of friction of the guidance system
Using Nook linear bearings in this application;
µ = Coefficient of Friction for lubricated
Nook Linear Bearings = .0013
(Refer to linear ball bearing engineering data.)
N = Load = 5000 pounds
F = µ x N
F = .0013 x 5000 lbs.
F = 6.5 lbs.
Therefore:
The Axial Force the screw must produce to move the load is 6.5 lbs.
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STEP
2 Find Average Travel Rate.
The average travel rate is determined by dividing
travel distance by travel time.
V average = D/t
D = distance = 36 inches
t = total time = 10 seconds
V avg. = D/t
V avg. = 36 in. / 10 sec.
V avg. = 3.6 in / sec. or 216 in/minute
Therefore:
the average travel rate is 216 in/min.
STEP
3 Find Maximum Travel Rate.
When considering critical speed, peak velocity
should be used. Using a basic triangular motion profile (acceleration
= deceleration with no constant velocity travel), the peak velocity
equals twice the average velocity.
V peak = 2 x V avg.
V avg. = 3.6 in / sec. or 216 in/minute
V peak = 2 x V avg.
V peak = 432 in/min
The Maximum Travel Rate is
432 in/min during the traverse of 36 inches in 10 seconds.
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STEP 4 Determine
total unsupported length.
Total Travel is given as 36 inches, but extra
screw length should be considered for travel nut, carriage, and
or any extra screw length for over-travel.
Based on the travel nut and attachment of
the nut to the carriage in this application, it is determined an
extra 4" of screw length will be required. (Refer to the dimensional
information of the particular nut used)
L total = 36 in + 4 in = 40 inches
The total unsupported length to be used for critical
speed and column loading calculations is 40 inches.
STEP
5 Determining end fixity.
The layout of the application shows that adequate
space is available to use an EZZE-MOUNT™ at each end. (See
end fixity definitions)
End Fixity = Type “C”
STEP
6 Select a screw based on the critical speed.
Use previously determined values with the
Critical Speed chart.
Max Travel Rate = 432 in/min
End Fixity = Type “C”
Length Between Bearing Supports = 40 inches
Based on the Critical Speed Chart, a 1"- 5
Acme Screw (1 inch diameter, 5 threads per inch) is selected.
STEP
7 Check Column Strength of screw.
Use previously determined values with the
Column Strength chart.
Load = 6.4 pounds
End Fixity = Type “C”
Length Between Bearing Supports = 40 inches
Based on the Column Strength Chart, the load is
within the column strength of this screw.
NOTE: If this were a vertical application,
the full 5000 pound load would be used. Also, under high acceleration
conditions, the inertia load must be determined and added to the
total load for column considerations.
STEP
8 Check the PV Value.
This relates the pressure load to the speed
of the nut. First find the actual “P” value based on
the calculation. Using the following formulas: |
