Worm Gear Screw Jacks Definitions and Technical Data
As a manufacturer of lead screws, Nook Industries stocks a wide selection of inch and metric ball, acme and trapezoid screws in long lengths. Jacks are not preassembled or stocked with standard length screws. Each jack is made to order based on travel length.
Nook Industries has the capability to manufacture long screws for special applications, limited only by the availability of raw materials. Rotating screw jacks may be assembled with a larger diameter lift screw for greater column strength. Jacks can be supplied with special pitch lift screws to change the jack operating speed.
Twin Lead Screws
Jacks can also be assembled with twin lead screws if required by the application. Contact the engineers at Nook Industries for availability.
Travel vs. Input Revolutions
The number of turns of the worm required to move one inch is a function of the worm gear ratio and the lead of the screw. The charts at the front of each section give the number of "turns of worm for 1" raise" for each jack. The motor rpm divided by this number is the linear speed of the jack lift shaft or travel nut. Conversely, the desired travel rate multiplied by the "turns of worm for 1" raise" equals the input rpm required.
Lead Accuracy and Matched Lead
Lead accuracy is the difference between the actual distance traveled versus the theoretical distance traveled based on lead. For example: A screw with a 0.5 inch lead and ±0.004" per foot lead accuracy rotated 24 times theoretically moves the nut 12 inches. 24 Revolutions × .500 inches per revolution = 12.000 inches of travel. With a Lead accuracy of ±0.004" per foot, actual travel could be from 11.996 to 12.004 inches.
The rolled thread ball screw, as employed in ActionJac™ products, is held within ±0.004" per foot lead error. The rolled acme thread screws used in our machine screw jacks have a typical lead accuracy of ±0.004" per foot.
When multiple jacks are used to move a load with precise synchronicity, lift shafts of similar lead accuracy can be factory selected and supplied as sets. Consult factory for matched lead set tolerances.
The input torque is the rotary force required at the input of the jack to generate an output force at the lift shaft. The product specification pages in our Worm Gear Screw Jacks Catalog show the torque necessary to raise one pound. This number multiplied by the load is the required input torque.
Due to static friction, starting or "breakaway" torque can be as much as two to three times running torque. If the load is moved horizontally, the force required to move the load will be lessened in proportion to the coefficient of friction of the surface along which the load is moved. In addition, the force needed to start, stop and hold the load (inertia loading) is provided by the jack. Jack sizing should consider all these forces. If an application calls for several jacks to be driven together in series, the first jack should be limited to three times the rated Maximum Input Torque, as listed in the Jack Selection chart for the particular selected jack. For multiple high lead ball screw jacks or belt/chain driven jacks contact Nook Industries for allowable input torque values. Multiple jacks driven in a series may require operation at reduced load.
Tare Drag Torque
The gear box components (bearings, seals and grease) in a jack add "tare drag". The product specification pages show the tare drag torque. When loading ActionJac™ Worm Gear Screw Jacks with loads less than 25% of their rated capacity, tare drag torque needs to be added to the torque requirement.
ActionJac™ Worm Gear Screw Jacks are rated for up to 3,000 rpm input speed, provided horsepower and temperature ratings are not exceeded. Contact Nook Industries engineers if higher input speeds are required.
Duty cycle is the ratio of run time to total cycle time. Some of the mechanical energy input to a worm gear screw jack is converted into heat caused by friction. The duty cycle is limited by the ability of the worm gear screw jack to dissipate heat. An increase in temperature can affect the properties of some components resulting in accelerated wear, damage and possible unexpected failure.
Maximum allowable horsepower ratings (see product specification pages) are based on intermittent operation. The approximate allowable duty cycles are: Ball Screw Jacks= 35% Machine Screw Jacks= 25%.
Self-Locking and Brakes
Self-locking occurs when system efficiencies are low enough that the force on the lifting shaft cannot cause the drive system to reverse direction. Machine Screw Jacks having gear ratios between 20:1 and 32:1, are self-locking and, in the absence of vibration, will hold loads without backdriving. All other ratios may require a brake to prevent backdriving. All Ball Screw Jacks can backdrive and require some means of holding the load, such as a brake on the motor. The product specification pages show holding torque values. Holding torque represents the amount of input torque required to restrain the load.
In addition to back driving, system inertia usually results in some over travel when the motor is switched off. The inertia of the system should be considered when determining the brake size required to stop a dynamic load.
All ActionJac™ Worm Gear Screw Jacks are suitable for operation within the specified limits provided that the housing temperature is not lower than -20°F or higher than +200°F. Factory supplied grease in standard units will operate in this range. For higher or lower operating temperature ranges contact Nook Industries.
Housing temperature should be monitored and kept below 200°F maximum. Continuous or heavy duty operation is possible by de-rating the jack capacity, external cooling of the unit or through the use of a recirculating lubrication system.
Travel stops are not standard. A limit switch and a brake should be used to stop the motor. Mechanical stops can cause damage to the jacks because most electric motors will deliver stall torques much higher than their rated torques and motor inertia can cause severe shock loads. For hand operation, mechanical stops can be provided.Figure 1