Machinedesign 1119 Translating Jack 0 0

Jack it up

April 1, 2000
In a world turning increasingly electrical, sometimes the best approach when you need power is mechanical

In an appropriate design and given proper maintenance, screw jacks reliably lift, position, support, or hold industrial loads for years. Incorporating them into an application is a fairly simple matter – needing primarily an understanding of available designs and application need.

Wind up (or down)

The basic components of screw jacks are a power screw, gearbox, and thrust-bearing load support. Jacks come in two basic configurations. In the keyed-for-traveling-nut design, the gearbox and screw are kept stationary with respect to each other while a bronze alloyed nut travels along the rotating screw.

With the more common translating screw jacks, a nut in the gearbox draws the screw through, providing linear motion. However, this action requires that the screw not rotate. The load can prevent rotation because it’s either fixed to a plate welded to the top end of the screw or threaded onto machined vee threads at the screw end. Alternatively, guides or additional jacks, which can also support a load, will stop unwanted rotation.

For single jack applications where the load can rotate, the keyed screw jack is another choice. This variation on the translating screw jack augments the design with a keyway machined along the screw and the key built into the jack body.

Screw jacks frequently use either machine screws (usually ACME threads) or ball screws, each with its own advantages. Machine screws are best for slow, infrequent motions because of their efficiency, which typically runs from 30 to 40%. Although this percentage may be considered low, it makes most machine screw jacks selflocking, eliminating the need for brakes and other external locking devices. Thus, inefficiency and low cost are the machine screw jack’s greatest strengths and make it a simple, safe, as well as cost effective solution to lifting loads.

Jacks built using ball screws, which are typically 90% efficient, can be operated faster and more frequently using less power. However, these screws are not self-locking.

Gear up

Screw jacks often use worm gears, which can range from 250 pounds to 250 tons capacity per jack, with two or three gear ratios available in each size.

When selecting motors to power these jacks, keep in mind that catalog efficiency ratings are given for a specific speed. Operating the jack at a lower speed will actually require more torque because worm gears are less efficient when they move slowly.

Spiral bevel-gears in screw jacks increase gearbox efficiency in rotating the internal nut or jack screw. Plus, they retain their self locking features because of the inefficiency of the machine screw.

Unlike other types of jacks, ball screw bevel gear jack life is highly predictable. The ball nuts have a B10 life, similar to that of a ball bearing. This indicator is arrived at through statistical calculations, and says that 90% of ball nuts will meet or exceed calculated life at a given load. In addition, the spiral bevel gears also have a predictable life.

Turn of the screw

To manually adjust a jack, it generally takes at least 24 turns of the input shaft for each inch of linear travel (defined as a function of the gear ratio and screw lead, and which varies from jack to jack). As the load increases, the input torque needed to move it goes up proportionally. Manually raising the load then requires additional gear reduction, which results in more turns per inch of travel. Thus, a good rule of thumb is to consider using a motor if the load is greater than two tons or the adjustment distance greater than two inches.

Most jacks are powered by ac motors, which offer high starting torque. Even though high starting torque is desirable, the main selection guideline is a jack’s running torque.

Determining reflected inertia in a screw jack is usually not necessary because the actual values are usually so small, they are considered insignificant.

Mount up

Mounting depends on a jack screw’s configuration and how it’s attached to a load and stationary frame.

Available space will limit your configuration choices. With a translating screw jack, the screw can mount underneath a load. The base usually mounts to a stationary frame. In a platform lift, such an arrangement allows a completely open top. However, a hole must exist below the jack body to accept the retracted screw.

In a keyed-for-traveling-nut version, one end of the jack can mount to a solid surface with the free end supported by a flange bearing. There will always be a rotating screw protruding from this version.

If the screw jack must travel through an arc, it would need a male clevis on its end. The base would mount to a custom pivoting platform. For short travel distances, a second male clevis can be mounted at the end of the protection tube (an optional pipe on translating screw jacks to protect the screw when retracted).

One jack is usually strong enough to raise a given load. However, supporting a load usually requires different treatment. Thus, most screw jack systems consist of several jacks connected through a series of shafts and miter boxes, which also help simplify movement synchronization. Bevel gear jacks make this task simpler because they can be placed in the corners, eliminating miter boxes and associated mounting plates.

Assuming that column buckling factors have been considered, applications moving only a small load (relative to the supported load) may be able to use a lower capacity jack. To move large loads, other factors such as travel speed and duty cycle come into play.

Putting it all together

In any mechanically linked jack system, three factors will affect total system performance: selection of system components, load distribution, and the effect of guides.

The prime mover, or motor, is perhaps the most crucial choice for a jack system. It must accommodate the additional drag that each coupling, miter box, and pillow block will add to the system. Sometimes, the increased drag is enough to boost horsepower requirements by 30% or more. Depending on each component’s location, system drag could require the use of larger shafts and gearboxes than originally determined.

Most structures are not rigid, so uneven load distribution always exists. This is also an important factor during the installation of a system where the relative positions of unloaded jacks may not correspond to the actual load on each jack.

A simple test for load distribution is to check the running temperature of each jack with the load moving. For example, in a four jack system, two jacks in opposing corners may run at 150° F while the other two are only at 80° F. It is safe to assume that the two running at higher temperatures are supporting the bulk of the load and the other two are simply providing balance. To change this situation, simply loosen some of the couplings and adjust the relative positions of the cooler jacks.

Another factor to consider in system design is the effect of guides. Frequently, guides are used to prevent transmission of a side load to the screw jack. A properly designed guide system will not add any significant load. However, a poorly designed or manufactured guide system could increase drag, driving up the power requirements of the system.

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