Screw Jack Design Considerations

Ball Screw vs. Machine Screw Jack

The decision to use a ball screw jack or a machine screw jack is based on the application. For many applications, a ball screw model is the best choice. Ball screw jacks are more efficient and therefore require less power than a machine screw jack in the same application.

For low duty cycle applications, for hand-operated applications, or if backdriving is not acceptable consider a machine screw jack.

ActionJac™ Ball Screw Jacks are preferred for:

• Long travel lengths
• Long, predictable life
• High duty cycles
• Oscillating motion

ActionJac™ Machine Screw Jacks are preferred for:

• Resistance to backdriving
• Vibration environments
• Manual operation
• High static loads

Jack Sizing Considerations

Jacks are limited by multiple constraints: load capacity, duty cycle, horsepower, column strength, critical speed, type of guidance, brakemotor size, and ball screw life. To size a screw jack for these constraints, application information must be collected.

Load Capacity

The load capacity of the jack is limited by the physical constraints of its components (drive sleeve, lift shaft, bearings, etc.). All anticipated loads should be within the rated capacity of the jack. Loads on the jack in most applications include: static loads, dynamic or moving loads, cutting forces or other reaction forces and acceleration/deceleration loads.

For shock loads, the peak load must not exceed the rated capacity of the jack, and an appropriate design factor should be applied that is commensurate with the severity of the shock.

For accidental overloads not anticipated in the design of the system, jacks can sustain the following overload conditions without damage: 10% for dynamic loads, 30% for static loads.

Total Load

The total load includes static loads, dynamic loads and inertia loads from acceleration and deceleration. Also consider reaction forces received from the load such as drilling or cutting forces when using a jack to move a machine tool.

For multiple jack systems, load distribution should be considered. System stiffness, center of gravity, drive shaft windup and lead variation in the lift shafts may result in unequal load distribution. Jacks of varying capacity with equal "turns of worm for 1" travel" may be used to accommodate unequal loading.

Number of Jacks

The number of jacks used depends on physical size and design of the equipment. Stiffness of the equipment structure and guide system will determine the appropriate number of jacks required. Fewer jacks are easier to drive, align and synchronize.

Duty Cycle

Cycle Time

Total time the jack is operating in one complete cycle

Duty Cycle

Percentage of time on versus total time.

Verify the duty cycle for the selected jack. Recommended duty cycles at max horsepower are:

• Ball screw jacks = 35% (65% time off)
• Machine screw jacks = 25% (75% time off)

The ability of the jack to dissipate the heat that builds during operation determines duty cycle. Anything that reduces the amount of heat generated or increases heat dissipation will allow higher duty cycles. Jacks may be limited by maximum temperature (200°F) and not duty cycle. Contact Nook Industries for assistance with these applications.

Horsepower Ratings

The horsepower limit of the jack is a result of the ability to dissipate the heat generated from the inefficiencies of its components. Maximum horsepower ratings are based on intermittent operation. Horsepower is calculated by using the following formula:

The product specification pages in our Worm Gear Screw Jacks Catalog show the "torque to raise one pound" value for each jack. Add tare drag torque if operating under 25% rated load.

Horsepower values are influenced by many application specific variables including mounting, environment, duty cycle and lubrication. The best way to determine whether performance is within horsepower limits is to measure the jack temperature. The temperature of the housing near the worm must not exceed 200°F.

For multiple jack arrangements, total horsepower required depends on horsepower per jack, number of jacks, the efficiency of the gear box(es) and the efficiency of the arrangement.

Arrangement Efficiency

• Two jacks = 95%
• Three jacks = 90%
• Four jacks = 85%
• Six to eight jacks = 80%

The efficiency of each miter gearbox is 90%. Therefore, motor horsepower requirement for the arrangement:

where N = Number of gearboxes

Do not exceed the maximum allowable input horsepower for a jack. Many models cannot lift the full rated load at 1,800 rpm.

If the horsepower required exceeds the maximum value for the jack selected, several solutions are possible.

• Use a larger jack model to increase the maximum allowable horsepower.
• Use a Ball Screw Jack to reduce the power required to do the same work.
• Operate at a lower input speed.
• Use a right angle reducer to bring the power requirement within acceptable limits.

When utilizing multiple jack arrangements, the input torque to the first jack must be considered. It is recommended that the number of jacks driven through a single jack input be limited to a maximum of three jacks. Consult Nook Application Engineers for arrangements where more than three jacks will be driven through a single jack input.

Column Strength

Column strength is the ability of the lift shaft to hold compressive loads without buckling. With longer screw lengths, column strength can be substantially lower than nominal jack capacity.

If the lift shaft is in tension only, the screw jack travel is limited by the available screw material or by the critical speed of the screw. Refer to the acme screw and ball screw technical sections for critical speed limitations. If there is any possibility for the lift shaft to go into compression, the application should be sized for sufficient column strength.

Charts are provided in each section to determine the required jack size in applications where the lift shaft is loaded in compression.

To use the charts, find a point at which the maximum length "L" intersects the maximum load. Be sure the jack selected is above and to the right of that point.

Maximum Length

The maximum length includes travel, housing length, starting/stopping distance, extra length for boots and length to accommodate attachment of the load.

If column strength is exceeded for the jack selected, consider the following options:

• Change the jack configuration to put the lift shaft in tension
• Increase size of jack
• Add a bearing mount (like the EZZE-MOUNT™) for rotating jacks
• Change the lift shaft mounting condition (e.g. from clevis to top plate)

CAUTION: Chart does not include a design factor.

The charts assume proper jack alignment with no bending loads on the screw. Effects from side loading are not included in this chart. Jacks operating horizontally with long lift shafts can experience bending from the weight of the screw.

Critical Speed

The speed that excites the natural frequency of the screw is referred to as the critical speed. Resonance at the natural frequency of the screw will occur regardless of the screw orientation or configurations of the jack (vertical, horizontal, translating, rotating, etc.). The critical speed will vary with the diameter, unsupported length, end fixity and rpm of the screw. Since critical speed can also be affected by the shaft straightness and assembly alignment, it is recommended that the maximum speed be limited to 80% of the calculated critical speed.

Because of the nature of most screw jack applications, critical speed is often overlooked. However, with longer travels, critical speed should be a major factor in determining the appropriate size jack. Refer to Nook Industries Precision Screw Assemblies Design Guide to best determine the appropriate critical speed for a particular jack selection.

Travel Rate

Establishing a travel rate allows for evaluation of critical speed and horsepower limits. Acceleration/deceleration time needs to be considered when determining maximum required travel rate.

Type of Guidance

Linear motion systems require both thrust and guidance. Jacks are designed to provide thrust only and provide insufficient guidance support. The guidance system must be designed to absorb all loads other than thrust.

Nook Industries can provide either hardened ground round shafting or square profile rail to support and guide linear motion systems.

Brakemotor Sizing

Safety is the most important consideration. A brake motor is recommended for all ActionJac™ products where there is a possibility of injury.

Only 20:1 or greater ratio Machine Screw Jacks can be considered self-locking in the absence of vibration.

The horsepower requirements determine the size of the motor. Upon selecting a brake motor, verify that the standard brake has sufficient torque to both hold the load and stop the load.

CAUTION: High lead ball screw jacks may require larger nonstandard brakes to stop the load. An appropriately sized brake will insure against excessive "drift" when stopping for both the Ball Screw and Machine Screw Jacks.

Ball Screw Life

A major benefit of the use of ball screw jacks is the ability to predict the theoretical life of the ball screw.

Ball screw life charts are located at the beginning of each ball screw jack section. 

Finishes

Per customer request, we can apply epoxy paint or MIL specification primers and paints or paint to other special requirements.