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SCREW JACK Duff-Norton line of screw jacks (mechanical actuators) includes mechanical actuators (machine screw and ball screw) ranging in size from 1/4 ton to 350 tons, stainless steel machine screw actuators, micro-miniature actuators, high duty cycle actuators, motorized machine screw and ball screw actuators, electric cylinders and motor control systems. Accessories include miter gear boxes, c-face motor adapters, limit switches, transducers, visual and digital position indicators, couplings, ball screws and ball screw systems, and locking rings. Screw Jacks are also used for raising and lowering maintenance platforms, opening and closing air dampers, oven doors and sluice gates, and adjusting tooling machine beds, cable tension and workplace tables. In addition to the broad range of applications and industries for mechanical actuators, they are also used as alternatives to hydraulic and pneumatic systems.
Screw Jack Selection Guide
1. Define the application's operating parameters:
  • Total load
  • Travel (distance to move the load)    
  • Load per actuator (if more than one is required)
  • Load type (tension, compression, guided, unguided)
  • Desired lifting speed
  • Ambient temperatures (-20º to 120ºF, -29º to 50ºC)

2. Determine which actuator type best suits the application:
Ball Screw or Machine Screw? There are a wide variety of factors which influence the type of actuator selected.  When comparing the two actuator types at the same capacity level; Ball Screw actuators, being much more efficient, require less motor horsepower to move the same load than do the equivalent Machine Screw actuators.  However, many Machine Screw actuators are inherently load holding, offer a broader capacity range and a greater selection of special features or materials. Machine screw actuators are often favored in applications subject to constant vibration.

Screw Jack: Ball Screw Actuator
Ball Screw Actuators
  • Continuous Duty Models available
  • Anti-Rotation Models Available, contact the factory for details
  • Move Loads and apply force more efficiently than machine screw actuators
  • Require less power by reducing screw friction
  • Permit faster operation and longer life under load
  • Long predictable ball screw and ball nut life
  • Handles full load in tension or compression

Screw Jack: Machine Screw Actuator
Machine Screw Actuators
  • Anti-Backlash models available for 1/4 to 150 ton capacities
  • Stainless Steel and Metric models available for most capacities

3. Calculate actuator performance:
Find an actuator model with Capacity greater than the actuator load. Go to the applicable Actuator Performance Specification table and find Turns of Worm for 1" Raise, Worm Torque at No Load, and Worm Torque at Full Load.
    A.  For loads greater than 25% of actuator capacity, consider torque to be proportional to load:

        Actuator torque(in-lb) = Actuator Load(lbs) x Worm Torque at Full Load
                                                                  Actuator Capacity (lbs)

For loads less than 25% of actuator capacity, add "Worm torque at no load" to the above calculated torque, to account for frictional losses.
    B. Calculate input RPM. Shaft input should not exceed 1800 rpm.

        Input RPM = Desired Lifting Speed(in/min) x Turns of Worm for 1" Raise 
    C. Calculate actuator input HP.

        Actuator Input HP = Actuator torque(in-lb) x rpm

Compare required Input HP to the Maximum HP per Actuator shown in the Performance table. If Required HP exceeds Maximum HP, an actuator with greater HP rating must be chosen to obtain the speed and capacity rating desired.
If using a gear reducer, motor horsepower must be multiplied by reducer efficiency to obtain reducer output (actuator input) horsepower.
    D. Multiple actuator arrangements:
Two or more actuators are often shaft driven from one motor or gear reducer. For multiple actuator arrangements, sum the input HP requirement of all actuators. If using mitre gear boxes, allow for 2% power loss through each 90° turn in the power path.

4. Determine the actuator configuration:
Considering capacity, speed, and duty cycle requirement, select the actuator type and configuration which most closely matches your application's  configuration requirements.

Images of an upright translating, inverted translating, rotating translating and rotating inverted actuator
          Upright Translating                  Inverted Translating                    Upright Rotating                   Inverted Rotating

5. Un-attached or un-guided load considerations:
If your application involves a load which is un-attached or the load is free to rotate, the translating screw actuator must be configured so that the lifting screw will extend when the actuator is in motion.  To prevent the translating screw from rotating, machine screw actuators are supplied with a keyed shell and screw, and ball screw actuators are supplied with a square nut on the lifting screw's end, inside a square cover pipe.  Both of these configurations ensure the actuator will properly perform for this type of application.

Image of an anti-rotating ball screw and keyed machine screw
                       Anti-Rotation Ball Screw                                                            Keyed Machine Screw

6. Verify the actuator selection:
Double check your application's travel requirements, and the actuator's ratio.  Verify the actuator's capacity and speed.  Also, determine which of the following actuator end fittings best suits your application's requirements.

Image different Screw Jack ends
                       Top Plate                                                   Clevis End                                              Threaded End

Please refer to our "Column Strength Charts" (pages 98 thru 102) if the lifting screw is loaded in compression. It may be necessary to select a larger actuator if the maximum recommended screw length, regardless of load, or maximum load has been exceeded.

As duty cycles are intermittent, there is an inverse relationship regarding an actuators maximum duty cycle and the load being moved. Please consult our application engineers for assistance in determining the most appropriate actuator.

  • Input RPM should not exceed 1800 RPM
  • Never exceed the actuator's static and dynamic capacity
  • Never exceed the horsepower listed in our actuator specification tables. If the maximum horsepower recommendation is exceeded, reduce the speed, use a larger capacity actuator, choose another actuator ratio, or consider a more efficient actuator type such as a Ball Screw or Continuous Duty Actuator
  • Ball Screw and Continuous Duty Actuators are inherently self-lowering. Should one of these models be the best fit for an application, a brake motor with sufficient torque is required. Please contact our application engineers for assistance.