8 Design Factors to Consider
Regardless of the type of worm gear jack you are considering (machine screw or ball screw), there are 8 factors that are critical for developing the optimal linear motion system. In this post, we will examine these design constraints and how they can impact proper sizing, placement and configuration of your screw jack system.
1. Load Capacity
The load capacity of the jack is limited by the physical constraints of the components including the drive sleeve, lift shaft, bearings. All types of anticipated loads must be calculated properly so they meet the rated capacity of the jack. These loads can include: static, dynamic, moving, acceleration/deceleration loads as well as cutting and other reactionary forces.
- It is critical that your design also accommodates shock loads. For optimal performance, these should not exceed the rated capacity of the jack.
- To accommodate accidental overloads, screw jacks can sustain the following overload conditions without damage: 10% for dynamic loads, 30% for static.
2. Duty Cycle
Duty cycle is the percentage of "time on" as opposed to "total time". The largest determining factor in calculating duty cycle is the ability of the screw jack to dissipate heat that builds up during operation. As the generated heat increases or decreases, the duty cycle will also be impacted accordingly. Additionally, screw jacks may be limited by their maximum operating temperature (200°F) and not duty cycle.
The recommended duty cycles for screw jacks at max horsepower are:
- Ball screw jacks 35% (65% off)
- Machine screw jacks 25% (75% off)
3. Horsepower Ratings
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 (Housing temperature near worm must not exceed 200°F.
The horsepower limit of a jack is a result of the ability to dissipate the heat generated from the inefficiencies of its components, based on intermittent operation.
- Special consideration should be given for multiple jack arrangements, as total horsepower required depends on horsepower per jack, number of jacks, the efficiency of the gear box or boxes and the efficiency of the arrangement.
If the required screw jack maximum horsepower is exceeded:
- Use a larger screw jack size
- Consider a Ball Screw Jack (as a Machine Screw Jack alternative)
- Decrease the input speed
- Use a right angle reducer
4. 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.
If there is any possibility for the lift shaft to go into compression, the application should be sized accordingly. Designers should also be aware of effects of side loading. Screw jacks operating horizontally with long lift shafts can experience bending from the weight of the screw.
If the maximum column strength is exceeded:
- Change the jack configuration to put the shaft in tension
- Use a larger screw jack size
- Add a bearing mount for rotating jacks
- Change the lift shaft mounting (Ex: from clevis to top plate)
5. Critical Speed
The speed that excites the natural frequency of the screw is referred to as the critical speed. Critical speed varies with the diameter, unsupported length, end fixity and rpm of the screw.
Due to the nature of most screw jack applications, critical speed is often overlooked. However, with longer travels, it should be a major factor in determining the appropriate size jack.
- 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.
6. Type of Guidance
All linear motion systems require both thrust & guidance. Worm gear jacks are designed to provide thrust only and a guidance system should be designed to absorb all loads other than thrust. Preferred systems include hardened ground round shafting or square profile rail.
7. Brakemotor Sizing
For proper safety, a brakemotor is recommended for worm gear jack screws where the possibility of injury exists. Horsepower requirements will determine the size of the motor, and assure the standard brake has sufficient torque to both stop and hold the load.
- High lead ball screws may require larger, nonstandard brakes to stop the load, to ensure against excessive “drift” when stopping.
8. 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. Most major manufacturers will provide life charts for their products.
Once these factors are fully understood and considered, they can optimize the features and benefits of Machine Screw Jacks and Ball Screw Jacks. This ultimately makes selecting the right screw jack for your application considerably easier.