FREQUENTLY ASKED QUESTIONS
Screw Jacks (Mechanical Actuators)
Click on a question from the list below for more details:
What is the Lifting Torque Required for a Screw Jack ?
The input torque for a single screw jack depends on the load, the worm gear ratio, type of screw (machine screw or ball screw) and the pitch of the lifting screw. Torque values are listed in the individual product specification charts based on capacity loads. For loads from 25% to 100% of screw jack model capacity, torque requirements are approximately proportional to the load.
What is The Maximum Input Power & Speed for a Screw Jack ?
The input power to the screw jacks should not exceed the power rating shown in the specifications table. Maximum rpm should not exceed 1800, however the high duty "Sym-metric" screw jacks can run up to 3000 rpm. Screw Jack cannot accept responsibility for the overheating and rapid wear that may occur should these limits be exceeded. Power increases in direct proportion to the speed, and the motor size will be out of proportion to the screw jack model design rating should the speed become excessively high. When selecting the maximum permissible speed for a screw jack arrangement, always check to see that the power rating of the screw jack model is not exceeded.
What is the Efficiency of a Screw Jack ?
Actuator model efficiencies are listed in the individual product specification charts.
What is the Expected Life of a Screw Jack ?
The life expectancy of a machine screw jacks lifting screw, nut and worm gear set varies considerably due to the extent of lubrication, abrasive or chemical action, overloading, excessive heat, improper maintenance, etc. For detailed life calculations consult Screw Jack Ltd.
Does the Input Torque of a Screw Jack Differ Between Translating and Rotating Screw Models ?
The input torque, as well as the efficiency and side load ratings, is the same for both translating screw and rotating screw jacks.
How Does The Screw Jacks Lifting Screw Operate ?
When a screw jack unit is operated, the rotation of the worm shaft causes the worm gear to rotate. For translating screw jacks the worm gear is threaded to accommodate the lifting screw thread; as the worm gear turns, the friction forces on the screw thread act to turn the screw also. The greater the load on the actuator unit, the greater the tendency of the screw to turn. It is obvious that if the screw turns with the nut (worm gear), it will not raise the load. In those cases where a single unit is used, and where the load cannot be restrained from turning, it is necessary to key the lifting screw. Lifting screw key torque (refer to Design Guide PJLMPTDG-01 section 1.5.1.5.) must be checked as excessively heavy unguided loads could break the key. Rotating screw jacks work in a similar manner except that the lifting screw rotates with the worm gear and the lifting nut translates along the rotating screw.
Can a Screw Jack be Supplied with a Lifting Screw to Prevent Rotation ?
For all machine screw jacks a keyed lifting screw is available. Note the keyway in the screw causes greater than normal wear on the internal threads of the worm gear.
For ball screw jacks the lifting screw cannot be keyed, as the keyway would interrupt the ball track, permitting loss of the recirculating balls. Instead the option of a square anti-rotation tube can be fitted to ball screw jacks to prevent rotation (refer to Design Guide PJLMPTDG-01 section 1.3.1.10.). For further details consult Screw Jack Ltd.
For Standard Screw Jacks How Do I Prevent The Load from Rotating ?
We recommend the following methods for preventing the rotation of the lifting screw on standard screw jacks. For multiple screw jack systems, bolt the lifting screw top plates to the common member being lifted all the units. For single screw jack applications, bolt the lifting screw top plate to the load and ensure the load is guided to prevent rotation. A guided load is recommended as a heavy unguided load could cause key failure.
Are Screw Jacks Self-Locking ?
Screw Jacks with 24:1 and 25:1 gear ratios are considered self-locking in most cases.
The following screw jack models are considered not to be self-locking:
All Metric and Imperial ball screw jacks.
- The M2555 (1/4 ton) screw jacks with 5:1 gear ratio.
- The E2625 (5kN) & M2625 (1/2 ton) screw jacks with 5:1 gear ratio.
- The E2501 (5kN) & M2501 (1-ton) screw jacks with 5:1 gear ratio.
- In some cases the E1802 (25kN), M1802 & M9002 (2 ton) screw jacks with 6:1 gear ratio. For advice on an application basis consult Screw Jack Ltd.
- In some cases the E1805 (50kN) & M1805 (5 ton) screw jacks with 6:1 gear ratio. For advice on an application basis consult Screw Jack Ltd.
- In some cases the E1810 (100kN) & M1810 (10 ton) screw jacks with 8:1 gear ratio. For advice on an application basis consult Screw Jack Ltd.
- In some cases the M1815 (15 ton) screw jacks with 8:1 gear ratio. For advice on an application basis consult Screw Jack Ltd.
- All screw jacks with double start lifting screws are considered not to be self-locking.
Screw Jacks considered not self-locking will require a brake or other holding device (refer to Design Guide PJLMPTDG-01 section 1.1.3.2.7.).
If vibration conditions exist, refer to Design Guide PJLMPTDG-01 section 1.5.2.1.4.5. However for detailed advice and analysis consult Screw Jack Ltd.
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Are Shock Loads Permissible on a Screw Jack ?
Shock loads should be eliminated or reduced to a minimum, if they cannot be avoided, the screw jack model selected should be rated at twice the required static load. For severe shock load applications, using the Sym-metric, Metric and Imperial screw jacks, the load bearings should be replaced with heat-treated steel thrust rings which is an option available from Screw Jack. Note this will increase the input torque approximately 100 percent.
What is the Backlash in a Screw Jack ?
Standard machine screw jacks, machine screw jacks with anti-backlash and ball screw jacks must be considered separately, as the normal backlash will vary due to different constructions.
Backlash in Standard Machine Screw Jacks
Machine screw jacks have backlash due not only to normal manufacturing tolerances, but to the fact that there must be some clearances to prevent binding and galling when the screw jack unit is under load (for values refer to Design Guide PJLMPTDG-01 section 1.5.1.8.). Usually, the backlash is not a problem unless the load on the screw jack unit changes between compression and tension. If a problem does exist, then a unit with the anti-backlash feature should be considered.
Screw Jacks with the Anti-Backlash Device
The anti-backlash device reduces the axial backlash in the lifting screw and nut assembly to a regulated minimum. As the backlash will increase as the lifting screw thread on the gear wears the anti-backlash device can be adjusted to remove this normal condition.
Ball Screw Jacks
Ball Screw jacks do not have an anti-backlash option similar to the machine screw jacks. Instead for zero or reduced axial play ball screw jacks can be ordered with a pre-loaded ball nut (refer to Design Guide PJLMPTDG-01 section 13.1.1.2.).
What is the Column Strength of the Screw Jack ?
The column strength of a screw is determined the relationship between the length of the screw and its diameter. Column strength nomographs are included in the "Linear Motion & Power Transmission Design Guide" PJLMPTDG-01 (refer section 1.5.1.1.).
What Side Loads are Permitted on a Screw Jack ?
Screw jacks are designed primarily to raise and lower loads and any side loads should be avoided. The units will withstand some side loads, depending on the diameter of the lifting screw and the extended length of the lifting screw. Where side loads are present, the loads should be guided and the guides, rather than the screw jacks, should take the side loads - particularly when long raises are involved. Even a small side load can exert great force on the housings and bearings and increase the operating torque and reduce the life expectancy. Side Load Rating Charts are included in the "Linear Motion & Power Transmission Design Guide" PJLMPTDG-01 (refer 1.5.1.6.)
What is the Maximum Raise or Working Stroke for a Screw Jack ?
Generally, standard raises / strokes are:
- up to 300mm on 5kN and 500mm on 10 kN on metric screw jacks.
- 12 inches on 1/4 and 1/2 ton models and 18 inches on the M2501 (one ton) imperial screw jacks.
- Larger Screw Jacks have their maximum raise / stroke available limited only the available length of bar stock from suppliers (note - special steel production runs can be organised for special applications). Practical lengths will be affected whether the screw is to be subjected to compression or tension loads. Depending on diameter the length can be limited due to deformation of material in the machining process or column strength of the screw when subjected to compression loads. Long raise applications should be checked with Screw Jack for the following:
a) Side loads on extended screw.
b) Column strength of screw.
c) Thermal rating of screw and nut.
Screw Jack recommend guides be used on all applications. The longer the raise, the more important this becomes.
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What is the Allowable Duty Cycle of a Worm Gear Screw Jack ?
Because of the efficiency of conventional metric and imperial worm gear screw jacks, the duty cycle is intermittent at rated load. At reduced loading, the duty cycle may be increased. The high duty "Sym-metric" screw jacks have higher thermal efficiencies due to their design allowing generally 50% higher duty cycles than conventional style screw jacks. For detailed analysis consult Screw Jack Ltd.
At What High Temperatures Can Worm Gear Screw Jacks Operate ?
Screw Jacks are normally suitable for operation at ambient temperatures of up to 90ºC. Operations above 90ºC will require special lubricants. For temperatures above 90ºC, the life of even special lubricants is limited. Therefore consult Screw Jack on your application. For temperatures above 90ºC, advise Screw Jack of full particulars of the duration of such temperatures. In some cases, it may be necessary to furnish an unlubricated unit, then the customer can supply the lubricant of his own choice. Screw Jack suggest that a lubricant manufacturer be consulted for type of grease and lubrication schedule. As a general rule, the screw jacks unit should be shielded to keep ambient temperatures to 90ºC or less.
Seals for temperatures above 120ºC are expensive. Instead, Screw Jack can substitute bronze bushings for seals in these cases. If bellows boots are used, special materials will be required for temperatures above 90ºC.
Screw Jack can manufacture special screw jacks for high operating temperatures above 120°C. Consult Screw Jack Ltd on an application basis.
At What Low Temperatures Can Worm Gear Screw Jacks Operate ?
With the standard lubricant and materials of construction, the screw jacks are suitable for use at sustained temperatures of -20ºC. Below -20ºC, low temperature lubricant should be used. Also, at temperatures below -20ºC, if there is any possibility of shock loading, special materials may be required due to notch sensitivity of the standard materials at lower temperatures. Screw Jack application engineers must be consulted in these instances for a recommendation.
Actuators with standard material of construction and lubrication may be safely stored at temperatures as low as -55ºC.
What is The Thermal / Heat Build-Up in a Screw Jack as it is Operated ?
The duty cycle, the length of the screw, the magnitude of the load, and the efficiency of the screw jack unit all have a direct influence on the amount of heat generated within the screw jack. Since most of the power input is used to overcome friction, a large amount of heat is generated in the worm gear set in both ball screw and machine screw jacks, and in the lifting screw of machine screw jacks. Long lifts can cause serious overheating. High duty "Sym-metric" screw jacks have oil lubricated cubic gearbox housing specifically designed to dissipate heat more efficiently with increased surface area and mass, allowing increased duty capabilities.
Can Continuous Duty Screw Jacks be Supplied ?
Recommendation should be obtained from Screw Jack on this type of application and a completed application analysis form submitted. In general, semi-continuous operation can be permitted where load is light as compared to screw jack rated capacity. Units so used should be lubricated frequently and protected against dust and dirt. High duty screw jacks such as the "Sym-metric" and "M7500" Series, are oil-lubricated and are designed for maximum duty cycles. Special designs of screw jacks fitted with ball screws or roller screws may also suit high duty applications, consult Screw Jack Ltd for details.
How Do I Mount Bellows Boots on an Inverted Screw Jack ?
Metric and Imperial inverted screw jacks with bellows boots must incorporate an allowance in the length of the lifting screw for both the closed height of the boot and structure thickness. Since Screw Jack can make no provision for attaching a boot on the underside of the customers structure, Screw Jack suggest that a circular plate similar to the lifting screw top plate be welded or bolted to the bottom of the structure supporting the screw jack, there making it possible to use a standard bellows boot. (refer to Design Guide PJLMPTDG-01 sections 1.2.1.8., 1.2.2.8., 1.2.3.7. and 1.3.1.3. and 1.3.2.8.).
"Sym-metric" cubic screw jacks allow mounting from two sides instead of one and allow mounting on the same side as the bellows boot with only an access hole required in the structure for the lifting screw and bellows boot.
Can I use Screw Jacks to Pivot a Load ?
A screw jack can be built to pivot a load two methods:
Double Clevis Screw Jack
The screw jack can be furnished with a clevis at both ends (commonly referred to as a double clevis screw jack). The bottom clevis is welded to the bottom end of an extra strong cover pipe, which is fitted to the base of the screw jack. This bottom pipe still performs its primary function of encasing the lifting screw in its retracted portion. See the double clevis model illustrations on the dimensional drawings (refer to Design Guide PJLMPTDG-01 sections 1.2.2.10., 1.2.3.5. 1.3.1.5. and 1.3.2.7.).
Clevis - Trunnion Mounting
The screw jack is fitted with the standard clevis end on the lifting screw and a trunnion mount adapter is bolted to the screw jacks base plate. For trunnion mount detail refer to Design Guide PJLMPTDG-01 section 1.2.2.12.
The design of the structure in which these type of screw jacks is to be used must be constructed so that screw jack can pivot at both ends. Use only direct compression or tension loads, there eliminating side load conditions.
Can Screw Jacks be Supplied with Corrosion Resistant Properties ?
Screw Jacks can be supplied with alternative materials and/or paint specifications for high corrosive areas. These options include stainless steel, chrome plating, Electro-nickel plating, epoxy paint, etc..
What Type of Lubricants do the Actuators Use ?
The standard screw jacks are grease (EP2) lubricated for the lifting screw and gearbox assemblies. The high duty "Sym-metric" screw jack is oil lubricated for the gearbox and grease (EP2) lubricated for the lifting screw. All screw jacks can be supplied with industry specific lubricants, such as food or nuclear grade grease.
Can Screw Jacks be used within Rigid Structures or Presses ?
Screw Jack recommend that the screw jack selected has a greater capacity than the rated capacity of the press or of the load capacity of the structure. We also recommend that a torque clutch or similar device be used to prevent overloading of the actuator unit. Unless these precautions are taken, it is possible to overload the screw jack without realising it.
Will the Screw Jack Drift after its Motor is Switched Off ?
The screw jack will drift after the motor drive is switched off unless a brake of sufficient capacity is used to prevent it. The amount of drift will depend upon the load on the screw jack and the inertia of the rotor in the motor. Due to different construction, the ball screw jack must be considered separately (refer to Design Guide PJLMPTDG-01 section 1.1.3.2.7). Machine screw jacks require approximately one-half as much torque to lower the load as they do to raise the load.
For machine screw jacks with no load, the amount of drift will depend upon the size and speed of the motor. For example, a 1500 RPM input directly connected to a screw jack without a load will give on average 35mm to 60mm of drift; a 1000 RPM input will give about 1/2 as much drift. Note that the drift varies as the square of the velocity (RPM). The drift of the screw jacks screw can be controlled using a magnetic brake on the motor. Variations of drift will also be seen if the motor drives the screw jack via a reduction gearbox.
Can Screw Jacks Operate where Vibration is Present ?
Screw Jacks will operate in areas with vibration, however the vibration may cause the lifting screw to creep or inch down under load. For applications involving slight vibration, select the higher of the worm gear ratios. Should considerable vibration be present, use a drive motor equipped with a magnetic brake, which will prevent the screw jack from self-lowering.
Can Screw Jacks be Supplied with an Emergency Stop Discs, Pin or Nut ?
To prevent over travel of the lifting screw a stop disc, pin or nut can be fitted to a screw jack that is hand operated. For motor driven units it is possible for the full capacity of the screw jack or even a greater force (depending on the power of the motor) to be applied against the stop, there jamming so tightly it must be disassembled in order to free it. It is recommended that external stops are fitted where possible, however they must only be used as a last resort (Note - limit switches are one possible solution to constrain screw jack movement safely - consult Screw Jack for system advice). Under ideal conditions where a slip clutch or torque limiting device is used, a stop pin or stop nut may be used - but Screw Jack should be consulted. Note that the standard stop disc used the bottom of the lifting screw on ball screw jacks prevents the ball screw from running out of the ball nut during shipping and handling, there preventing loss of the recirculating balls. It should not be used as a full power stop.
Can Screw Jacks be built into Multiple System Arrangements ?
Perhaps the greatest single advantage of Screw Jack screw jacks is that they can be linked together mechanically, to lift and lower in unison. Typical arrangements involving the screw jacks, bevel gear boxes, motors, reducers, shafting and couplings are shown in application section of the web site and in the Design Guide PJLMPTDG-01 section 1.1.3.2.8.
How Many Screw Jacks Can be Connected in Series ?
This will be limited input torque requirements on the first worm shaft in the line. The torque on the worm shaft of the first screw jack should not exceed 300% of its rated full load torque on the machine screw jacks (this does not include the 1820 unit).
What is the Efficiency of a Multiple Screw Jack System ?
In addition to the efficiencies of the screw jacks and the bevel gearboxes, the efficiency of the screw jack arrangement must be taken into consideration. The arrangement efficiency allows for misalignment due to slight deformation of the structure under load, for the losses in couplings and bearings, and for a normal amount of misalignment in positioning the screw jacks and gearboxes. For efficiency values refer to Design Guide PJLMPTDG-01 section 1.1.3.2.8.
Can a Screw Jack be Equipped with a Position Indicator?
A visual position indicator for a screw jack can be provided in several ways, for example:
- Screw Jack with encoder and digital position indicator (refer to Design Guide PJLMPTDG-01 sections 8.3. and 8.4.)
- Screw Jack with rotary limit switch and position transducer (refer to Design Guide PJLMPTDG-01 section 8.1.2.)
However, it is suggested that you consult Screw Jack for recommendations based on your particular application.
