Bevel Jacks Design Considerations
Jacks are not pre-assembled 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.
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 show the torque necessary to raise per one kN. This number multiplied by the load is the required input torque. Due to static friction, starting or â€œbreakawayâ€ 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.
Bevel Gear Screw Jacks are rated for up to 3,000 rpm input speed, provided kilowatt and temperature ratings are not exceeded. A speed-factor must be applied when input speeds are greater than 1500 rpm. 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 Bevel Gear screw jack is converted into heat caused by friction. The duty cycle is limited by the ability of the Bevel 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. Consult efficiency diagrams for efficiency speed/load graph to best determine the proper duty cycle for your given application.
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. All bevel screw jacks can back drive 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 Bevel Gear Screw Jacks are suitable for operation within the specified limits provided that the housing temperature is not lower than 0Â°C or higher than 90Â°C. For higher or lower operating temperature ranges contact Nook Industries.
Housing temperature should be monitored and kept below 90Â°C maximum. Continuous or heavy duty operation is possible by derating 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.
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 back driving is not acceptable consider a machine screw jack.
Bevel Gear Ball Screw Jacks are preferred for:
- Long travel lengths
- Long, predictable life
- High duty cycles
- Oscillating motion Bevel Gear Machine Screw Jacks are preferred for:
- Vibration environments
- Manual operation
- High static loads
JACK SIZING CONSIDERATIONS
Jacks are limited by multiple constraints: load capacity, duty cycle, kilowatt, 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.
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.
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.
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.
The kilowatt limit of the jack is a result of the ability to dissipate the heat generated from the inefficiencies of its components. Kilowatt is calculated by using the following formula:
The product specification pages show the "torque to raise one kN" value for each jack.
Kilowatt values are influenced by many application specific variables including mounting, environment, duty cycle and lubrication. The best way to determine whether performance is within kilowatt limits is to measure the jack temperature. The temperature of the housing near the input shaft must not exceed 90°C.
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 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 â€“ 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 design factors. 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.
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.
Establishing a travel rate allows for evaluation of critical speed and kilowatt 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.
Safety is the most important consideration. A brake motor is recommended for all Bevel Gear products where there is a possibility of injury.
The kilowatt 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 â€œdriftâ€ when stopping for both the Ball Screw and Machine Screw Jacks.
The input shafts can either rotate CW or CCW with respect to extending the lift shaft. Care must be taken when using multiple jack arrangements. Bevel Jacks with two input shafts will rotate in opposing direction with respect to their common axis. See page XXX for multiple jack arrangements.
The bevel jacks are oil filled and are fitted with an oil vent. The bevel jacks come with four port plugs located on the same surface. When ordering, specify which surface the ports are to be located. After installation, replace the upper most port plug with the supplied oil vent.
Unlike standard worm screw jacks, Nook bevel screw jacks are oil filled. This requires venting to relieve pressure that can build up during the operation of the jack. The bevel jacks come with four port locations on a common face, which is typically located opposite the input shaft. The jacks will be shipping oil filled with one oil vent and four oil plugs. Once installed remove the upper most port and replace with the supplied vent.Figure 1