Machine Screw Jack and Machine Screw ComDRIVE® (.pdf file)
Stainless Steel Jack (.pdf file)
Metric Jack (.pdf file)
Bevel Gear and Bevel Ball (.pdf file)
Ball Screw Jack and Ball Screw ComDRIVE®(.pdf file)
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Column Loading Capacity
The type of load on a screw jack, and the way the jack is mounted, affects
its load bearing capacity.
There are two types of possible jack loads, tension and compression. A jack
is under tension when its load pulls the screw away from the jack, while
compression loads push the screw toward the jack (see first two illustrations
to left). A screw jack can be under tension or compression regardless of
jack positioning, i.e. vertical, horizontal, upright or inverted.
When tension loaded, the jack retains full rated capacity. Under compression
loads, however, the screw may not be able to support full capacity. For
example, a 2-ton jack with a 15" screw length will be limited to 2293 pounds
in compression, about half of the jack's capacity. In compression situations,
the load, screw length and jack mounting configuration determine the column
strength of the screw. The examples shown illustrate four common mounting
configurations.
Unguided
If the screw is the only support for the load, it is considered unguided. The
screw must be large enough to support the load and prevent buckling. On the
Column Strength load charts (links below illustrations), use the row labeled
"unguided" for the allowable lengths for this design.
Trunnion Mounting
In a trunnion mounting arrangement, the screw has a pivot on the end and the
jack body is mounted on a large pivoting frame, or trunnion. This type of
mounting is particularly common in the antenna industry. In practice, the
pivot should be as close to the centerline of the ACME nut as the design
permits. This will eliminate moment loads caused by loose threads. Use the
"trunnion" row on the charts for this mounting arrangement.
Guided
Guided loading is often termed "fixed-fixed" loading. Both ends of the column
are rigidly held - the jack body is bolted firmly to a sturdy base, and the
load travels on slides, bearings, rollers or other means. The guides should
be snug enough to prevent any side load or moment load from reaching the
screw. Use the "guided" row in the Column load charts.
Double-clevis Mounting:
Double-clevis screw jacks have less load capacity than the other common
mounting configurations. A double-clevis jack has pivots or clevises at both
ends: one on the screw tip and one on the end of the protection tube. This
tends to weaken it as a column by creating eccentric loads on the screw. This
eccentricity will tend to increase with greater distance and higher loading.
For this reason, double-clevis jacks are severely limited both in capacity
and maximum length. Double-clevis mounting differs from trunnion mounting
because the pivot is located further from the jack body. The column load
charts do not apply for this mounting, please consult the factory for load
bearing information.
How to use the Column load charts:
Note: Charts can be used for Machine screw jacks, Ball screw jacks, Stainless
steel jacks, Bevel Gear® jacks and Bevel Ball actuators. Charts only apply to
jacks with axial loads. For side loads, horizontal applications and offset
loads, contact the factory.
- Determine the proper mounting arrangement for your application. Locate the
appropriate row and find the screw length at the bottom of the chart.
- Find the load you need to move (in pounds) on the left side of the chart.
- Find the point on the chart where the load and length intersect. Choose a
jack whose line is on or above this intersection.
- Add the length of the end condition you have chosen and any additional
screw extension to the screw length to find "unbraced" screw length.
Verify your selection using the unbraced length.
Example:
A screw jack must lift 5 tons (10,000 pounds) over a distance of 31 inches.
The load places the screw in compression. The jack will be mounted firmly by
its base, and the load will be attached to a load pad (Type 2 end) and is not
guided.
- Look at the "unguided" row at the bottom of the Machine Screw Jack
Column Load Chart and find the 31" mark.
- Find the 10,000 lb. mark on the vertical axis (left side of the chart).
Move to the right along this line until it crosses the 31" line. Any jack
above this point should be suitable.
- From this, the 10-ton double lead screw jack is selected. Look at the
dimensions from the jack body for the Type 2 end for this jack. The Type 2
end adds 2" from the top of the jack to the end of the screw. Thus the
total unbraced length of the screw is 33 (31 + 2).
- Use this new unbraced screw length to verify your selection. In this case,
the intersection point still falls below the 10 ton double lead screw jack
line, so this selection is correct.
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