JACTON SCREW JACKS

Screw Jack FAQs

What is axially translating screw jack?

A driven worm shaft acts on an internal worm gear, which in turn drives a lifting screw to

extend or retract. As the lifting screw translates through the jack housing, inherent screw

rotation is prevented by an attached load, full guidances or mounting structure that is

anchored to resist rotation. The lifting screw will move linearly through the housing to push or

pull the load.

What is anti-rotation keyed screw jack?

The lifting screw of a translating screw jack must be attached to the load which prevents the

lifting screw from rotating. Otherwise, the lifting screw and the load will turn and not translate.

A keyed screw jack can prevent lifting screw rotation, which has milled keyway along the

length of  the lifting screw. A matching key is fastened to the cover of the jack, which will

eliminate jack screw rotation. The keyway in the screw causes greater than normal wear on

the internal drive worm gear threads, somewhat reducing screw jack lifespan. Note: JTB ball

screw jack, JTM machine screw jack, JTS bevel gear jack anti-rotation devices by square

protective pipe with square nut.

What is travelling nut rotating screw jack?

A keyed for travelling nut rotating screw jack features a lifting screw keyed to the internal

worm gear as a single unit, forcing the lifting screw to rotate, but not translate. A travelling nut,

attached to the load, is driven by the rotation of the lifting screw. This type of jack is ideal for

applications that cannot accommodate a screw protective pipe or that require a flush mount. 

What is the difference between an acme screw jack and a ball screw jack?

The acme screw jack uses an acme threaded screw that is completely self-locking, meaning it

will hold its position without a brake mechanism or locking systems. Ball screw jack use ball

screws to convert rotary motion to linear movement, and require approx. 1/3 the horsepower

compared to a acme screw jack. Due to the efficiency of the ball screw, brake mechanism

must be used to stop and hold the load screw in position. Brake mechanism is also

recommended for use on any screw jack if vibration is present.

Why use an acme screw jack and a ball screw jack?

One type of screw jack is usually better suited to the operating conditions. Typically, fast travel

speed and frequent cycle times may be more suited to a ball screw jack, particularly as the

load approaches the rated capacity of the screw jack. High load at slow travel speed, less

frequent cycles and the ability to hold the load in position when the system is at rest may be

better suited for an acme screw jack.

How do I operate the screw jack?

Most screw jacks are operated by electric motor, but air motors and hydraulic motors can also

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JACTON SCREW JACKS

be used. Hand wheel or crank handle can be used for manual operation.

Can I use a larger motor than required in specifications?

Yes, but it is not recommended. The screw jack or jack system components could be

damaged if an oversize motor is used. Travel limit switches control must be used for each end

of travel to stop the motor. If using solid mechanical stops, screw jack components can be

subject to shock load conditions and oversize motors can cause catastrophic failure of stops

and other components.

How do I stop the screw jack at the travel limits?

Limit switches or other controls must be used to shut off the motor when the screw jack has

reached its full extended or retracted position. Solid mechanical stops are not recommended.

Their continued use can cause severe damage to the screw jack. 

Can different size screw jacks be used in the same system?

Yes, as long as the equal travel length per full turn of worm shaft input. This is sometimes

done to accommodate varying load conditions.

What is the screw jack system efficiency?

2 screw jack system efficiency is 95%, 3 screw jack system efficiency is 90%, 4 screw jack

system efficiency is 85%, 6 screw jack system efficiency is 80%. Bevel gearbox efficiency is

98%. Single reduction helical gear reducer efficiency is 98%, Double reduction helical gear

reducer efficiency is 97%. Worm gear reducer efficiency is horsepower out x 100 /

horsepower in

What is the maximum input speed?

Jacton screw jack can be run at 1500Rpm. A gear motor, helical gear reducer, or a worm gear

reducer is used to reduce the input Rpm to the screw jack to provide the required travel speed

of the lifting screw. Many Jacton screw jacks can be driven directly by 1500Rpm and 900Rpm

electric motors. Electric motors and reducers are available mounted directly to many Jacton

screw jacks models.

What causes heat build up in the screw jack?

The screw jack is a mechanical gearbox assembly. The friction of the worm and worm gear,

lifting screw and worm gear, bearings and seals generate heat while the screw jack is

operating. The combination of travel, loading, and input speed all affect the temperature rise

of the screw jack. Jacton screw jack sizing calculations take these variables into account to

insure that you select the right screw jack model for your application.

What is the screw jack load capacity and travel?

This is based on the relationship of the lifting screw diameter and the length. Lifting screws in

tension are rated for the full capacity of the screw jack, regardless of length, providing they do

not rotate faster than the critical speed. For the lifting screw in compression, capacity is

limited by the screw jack's permissible buckling load. The permissible buckling load of a lifting

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JACTON SCREW JACKS

screw is reduced as the lifting screw gets longer. Use the maximum extended lifting screw

length when using the permissible buckling load charts to determine screw jack load capacity.

How do I determine the full-extended lifting screw length?

The extended lifting screw length is normally equal to the travel stroke. If screw jack is

equipped with bellows boot and stop nut, must be added the bellows boot full retract length

and the addition stop nut or special closed height. These must be increased the extended

lifting screw length. For inverted screw jack, the thickness of the mounting structure must be

included. This total extended lifting screw length should be used when determining the

permissible buckling load of the screw jack.

Should the load being positioned be guided?

It is highly recommended that the load be guided, however, it is not necessary. A guided

system will provide more column stability and allow longer screw jack travel. Column length is

greatly reduced on unguided systems. External load forces common with unguided systems

are detrimental to the life and operation of the screw jack.

Can the screw jack withstand side loading or a bending moment?

Yes, but this is not recommended. Consult Jacton if this condition will be present. These types

of loads apply greater forces on the screw jack and housing assembly causing premature

wear. Guides are highly recommended and should be used to eliminate side and bending

loads.

Can the screw jack withstand shock loading?

This is not recommended. Oversized screw jacks are required to handle shock loads. Solid

thrust bearings are also available in many screw jack models when constant vibration and

shock are present in an application.

What is the difference between upright and inverted screw jack configurations?

The difference between an upright and an inverted screw jack is the location at which the

lifting screw exits the jack relative to the jack gearbox. For example, an upright screw jacks

lifting screw exits the jack opposite the gearbox. An inverted screw jacks lifting screw exits the

jack on the same side as the gearbox. The choice between inverted and upright screw jack is

dependent upon the application.

Note: An upright screw jack mounted upside down is still referred to as an upright screw jack.

How is the linear travel speed calculated? 

Each screw jack has an inherent number show lifting screw travel per full turn of worm shaft

input. The result of the screw jack is equal to the lifting screw pitch divided by worm gear

ratio. For example, JT-10T model screw jack, screw pitch 8mm, high worm gear ratio 1/8, the

lifting screw travel is 1mm per full turn of worm shaft input. If 900Rpm is applied to the worm

shaft, the linear travel speed is 900Rpm multiplied by 1mm/r, equal to 900 millimeter per

minute.

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JACTON SCREW JACKS

Are screw jacks lubricated prior to shipment? 

All Jacton worm gear screw jacks and ball screw jacks are lubricated with an extreme

pressure NLGI grade #1 or 2grease before leaving the factory. Bevel gear screw jacks are

lubricated with NLGI grade #1 grease or VG220 gear oil. The upper bearing and jack screw

are grease lubricated while the remaining internal components are oil lubricated. They are

grease lubricated prior to shipment.

What electric motors options are available?

Electric motor options vary among product lines. Customers can use AC 3-phase motor, AC

single-phase motor, DC motor. Standard voltage of China is 380V/220V, 3-phase, 50Hz, or

220V, single-phase, 50Hz. Customized international voltage motors like America 110V-single

phase-60hz, 240V/480V-three phase-60Hz, other voltages 230V/240V-single phase-50hz,

400V/415V/440V-three phase-50hz etc.

Are screw jacks corrosion resistant?

Stainless steel screw jacks are inherently corrosion resistant. All exposed surfaces are

stainless steel and aluminum bronze. Most other screw jacks can be modified with special

finishes, coatings, and seals.

Can bellows boots be supplied for an screw jack model with inverted screw?

Yes, but allowance must be made in the length of the lifting screw for both the closed height of

the boot and structure thickness. Since we can make no provision for attaching a boot on the

underside of your structure, we suggest that a circular plate similar to the lifting screw top

plate be welded or bolted to the bottom of your structure supporting the screw jack, thereby

making it possible to use a standard bellows boot. 

Is there backlash between the lifting screw and worm gear?

Yes. This is necessary to allow for sliding or rolling action of the lifting screw through the worm

gear. Anti-backlash nut screw jack is available when the backlash needs to be minimized.

Input torque requirements are greater for anti-backlash nut screw jack.

How is the lifting screw protected?

Standard translating screw jack is fitted with a lifting screw protective pipe that stores the

lifting screw when the screw jack is in the closed position. Bellows boot is available and

recommended to protect the lifting screw in the extended position. Double bellows boots are

required for rotating screw jack with traveling nut.

How do I attach the load to be positioned to the lifting screw?

For translating screw jack, the lifting screw has a standard threaded end that can be used to

attach the load, top plate, clevis end, plain end and forked head are also available. For

traveling nut design, the traveling nut has a flange with mounting holes. The screw jack

gearbox has a mounting base and is also available with a clevis mount for double clevis

requirements.

Jacton Electromechanical Co.,Ltd | Tel: +86 (0)769 81585852 | Fax: +86 (0)769 81620195

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JACTON SCREW JACKS

Will the lifting screw rotate?

Yes. You need to prevent the lifting screw from rotating to produce linear motion. The lifting

screw will not rotate in a multiple screw jack system with all the lifting screws attached to the

same structure. If rotation of the screw cannot be prevented in the application design, a keyed

screw configuration is available for either the acme screw jack and ball screw jack. Input

torque will increase for keyed screw jack and the capacity may be reduced to 25% of rating.

What is the maximum and minimum operating temperature?

Standard Jacton screw jacks are designed to operate at maximum 80 deg.C (185 deg.F)

ambient temperature. For higher temperatures, special grease and seals are required, contact

Jacton of these special requirements. Minimum minus 20 deg.C (minus 4 deg.F) ambient

temperature. For lower temperatures, special grease and seals are required, contact Jacton

of these special requirements.

How do I lubricate a screw screw jack?

Use the proper grease. Fill the gearbox by pumping grease into the grease fittings supplied in

the screw jack housing. The lifting screws should have grease applied directly to them with a

rag or paintbrush. This must be done as part of a regularly scheduled maintenance program.

Jacton Electromechanical Co.,Ltd | Tel: +86 (0)769 81585852 | Fax: +86 (0)769 81620195

Email: sales@screw-jack.com | Web: www.screw-jack.com | Skype: jactonjack