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.
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.