By: Paul Hohensee, Contributing Author
The rigger picks up 100-ton jacks and hefts them into place
to set a bridge section, thinking about how it used to be. The waiting for
lifting equipment and help. The pushing and prying. What changed? The weight of
his jacks went from 130 lb to one-third that, because his new ones are made
from high-strength aluminum.
Way back in 1663, Pascal described the transmission of fluid
pressure from point to point and its application to the hydraulic jack. Ever
since, jacks have served to lift and move bridges and structures.
The very simplicity of the device has limited innovations to
details such as better seals, surface hardening and variations in size, shape
and accessories. Generation after generation, one thing remained constant: the plunger
and housing were machined from steel.
Steel reigned supreme as the basic material of jack
construction until about a decade ago, when aluminum cylinders appeared on the
scene. The aluminum cylinders offered considerably lighter weight, but like the
first generation of any new product, they had their drawbacks. The benefits of
aluminum's lighter weight were reduced by the lower material strength of common
alloys as compared with steel. The lower material strength meant thicker parts,
which offset some of the weight advantage and also made aluminum jacks bulky.
Weight-loss program
Jacks have followed the same pattern as beverage cans and
cars. First, aluminum was introduced as a replacement for steel. After that,
attention was devoted to reducing bulk and cutting more weight. Computerized
design, finite element analysis and modern alloys have made quite a difference.
The 7075T-6 alloy provides strength that rivals steel. The result: aluminum
jacks are now available in weights as low as one-third that of their steel
counterparts, yet their physical size is virtually the same as for steel.
The lightness of aluminum cylinders provides a two-fold
benefit: not only are they easier to handle, but also the jack size that can be
handled manually by one or two workers is doubled. That's significant in light
of the increased safety margins that may be required by DOTs in some states. In
those states, a lift-point that requires, for example, a 50-ton capacity may
have to be served by a 100-ton jack. Manually placing a 100-ton aluminum jack
is no problem; steel can be another story.
Properly designed aluminum cylinders are corrosion
resistant. In fact, they are selected for use in drydocks because of their
resistance to salt water. In certain situations, the non-magnetic properties of
aluminum are of special interest. In those cases, the ancillary steel parts can
be replaced with aluminum versions to create a non-magnetic jack.
Do the math
When considering procurement of hydraulic lifting equipment,
life-cycle cost analysis is the key to spending the least money. Upfront, steel
cylinders cost considerably less than aluminum, but that's only the first
chapter of the story. Purchasing justification should include a brief
explanation of differences in total cost over the life of the equipment.
Here's an example: A purchase of 150-ton jacks is being
considered. It is estimated that each jack will be placed 1,000 times during
its lifetime. If the steel version requires an additional $50 of labor compared
to the aluminum model each time it is placed; then the lifetime labor usage
cost of the steel cylinders is $50,000 higher than the aluminum.
Don't agree with the numbers? Use your own. The savings are
almost certain to be dazzling, even when more sophisticated analysis includes
factors such as the time value of money.
What to look for
Today's aluminum jacks may offer a far more attractive
alternative to steel than the limited selection of bulky models available a
decade ago. But there's more to aluminum hydraulic cylinders than the aluminum.
In order to cover your needs without having to use multiple sources, look for a
supplier that provides:
* Lifting range: A 20- to 150-ton range covers the realm of
manually placed cylinders;
* Type selection: Availability of single- and double-acting,
hollow plunger and lock nut versions assures ability to meet project
requirements;
* Stroke range: Availability of a wide stroke range, such as
2 to 10 in. for each capacity and type, covers most requirements;
* Surface hardening: Plunger and housing should be both
hardened and anodized to resist wear and galling;
* Stop rings: Cylinders should be equipped with stop rings
to positively prevent over-extension;
* High-strength alloy: Only cylinders made from
high-strength alloy deliver the full benefits of aluminum construction;
* Composite bearings: These prevent metal-to-metal contact,
reduce side-load problems and increase life;
* Steel base-plate: For protection from abrasive surfaces, a
replaceable steel base-plate should be supplied with each cylinder;
* Removable saddle: A removable, hardened steel saddle
handles wear caused by use on concrete and other abrasive surfaces; and
* Compliance with ASME B30.1: This is the design and test
specification for jacking cylinders. (Jacking cylinders are not to be used for
production or other "high-cycle" uses.)
The interest in lighter weight-lifting cylinders has not
only created better designs, but also the need to expand these lightweight
advantages into additional cylinder styles. Initially, aluminum cylinder
designs were limited to solid-plunger, spring-return models. This limited the
use of these cylinders to all but the most basic jacking applications. Today,
the trend for lighter weight cylinders has created the need for more complex
designs such as hollow, lock-nut and double-acting.
Hollow cylinders feature a center-tube to allow a threaded
bar or "stressing-bar" to be inserted through the center of the
cylinder. This bar is then mechanically capped off, allowing the cylinder to
"pull" against the cap, pulling the bar when pressurized, putting it
in tension. This is a typical application for anchor-bolt and pier testing or
strand tensioning.
Lock-nut cylinders feature a plunger with a high-strength
male thread, which mates to a locking collar or "lock-nut." When the
cylinder is extended into position, this lock-nut can be threaded down to the
top of the cylinder, providing a mechanical means to support the load. This
threaded nut is capable of supporting the entire capacity of the cylinder. The
load can now be supported for extended periods of time without any hydraulic
pressure applied to the cylinder. Typical applications are lifting bridge
decks--mechanically "locking" the load while maintenance is done and
then hydraulically lowering the deck when completed. Some users specify this
cylinder type in order to provide mechanical load support in case of a
hydraulic failure.
Lastly, the double-acting models feature a "hydraulic
return" in place of load return or spring return. This provides a faster,
more positive return over the other types, but typically these are slightly
taller and more costly due to additional seals required. These cylinders also
will require a slightly more complex hydraulic system because additional hoses
and valving will be required. Their advantage is in large jacking systems that
may require long hose runs, in which a spring return may not be strong enough
to push the oil back through the longer hose lengths.
For those willing to make the initial investment, aluminum cylinders
offer the most economical long-term solution to pushing, pulling or lifting
applications.
About The Author: Hohensee is construction market leader for the Americas at Enerpac, Milwaukee, Wis.
