How to choose your Drive System quickly

1. Pneumatics, hydraulics, or Screw Jacks: A lifting movement can be accomplished by three fundamental types

In essence, a linear movement might not seem very complex. At first glance, transporting an object from point A to point B doesn’t appear particularly challenging. However, there are significant differences in choosing a linear drive technology, which can greatly impact the effectiveness of your system. The primary systems are hydraulics, pneumatics, and electromechanics (e.g., screw jacks, spindle jacks, screw drives).

1. Hydraulic drives typically use hydraulic oil as the power-transmitting medium. A pump builds up pressure in a cylinder, which then moves the cylinder forward or backward.
2. Pneumatic drives operate on the same principle, but instead of oil, they use compressed air. A pumping unit continuously generates the compressed air.
3. Electromechanical drive technology usually converts rotary motion into linear motion. Its operating principle is based on sliding or rolling friction.

2. Hydraulic Drives

Hydraulics generally excel when large forces need to be moved with high duty cycles and speeds. The acquisition cost for a single cylinder is relatively low if a pump and peripheral equipment are already in place. Since hydraulic drives operate with pressure ratios, they can handle harsh operating conditions with impacts without issues.

However, for lifting systems with multiple cylinders, the complex control needed for pressure equalization can be a drawback. If the load on a lifting platform is unevenly distributed, the cylinders extend at different speeds unless proper control systems are in place. Additionally, the risk of leaks and maintenance cycles result in higher operational costs compared to electromechanical drives. While hydraulic cylinders have high efficiency, the pump for pressure generation must maintain pressure 100% of the time, consuming energy continuously.

3. Pneumatic Drives

If you exclude all operational and follow-up costs, you might think that pneumatic cylinders are a very cost-effective choice. The cylinders themselves are indeed a very economical solution compared to electromechanical drives, especially if compressed air is already available in the system. However, if this system is introduced in a new setup, it's important to consider that generating compressed air is one of the most expensive methods of energy transfer. There are also significant losses due to unwanted air leaks and the necessity of keeping the air generation unit constantly running.

Pneumatic cylinders are still widely used in many industries. The advantages, such as the ability to achieve extremely high speeds coupled with 100% duty cycle, outweigh the mentioned disadvantages. Additionally, the medium—air—does not cause contamination if it accidentally escapes into the environment.

4. Electromechanical Drives

Screw jacks, electric linear actuators, screw direct drives, and linear chains are systems that convert rotational motion into linear motion. In these systems, sliding and rolling friction play a crucial role in force transmission. In the field of safety technology, the physics of sliding friction is particularly important—standard trapezoidal lead screws in screw jacks typically achieve static self-locking. This self-locking capability refers to the ability to maintain the load at the same level even in the event of a power failure.

Another advantage, especially with screw jack systems, is the easy coupling of multiple jacks. By using coupling elements such as couplings, connecting shafts, universal joints, and distribution gears, torque can be efficiently transmitted to the correct location with minimal effort, all driven by a single motor. The ability to hold the load without power makes the system an energy saver over its lifetime, despite the relatively poor efficiency of screw jacks.

However, what benefits safety technology can lead to higher energy demands. With an average efficiency of 30% for screw jacks with trapezoidal lead screws, 70% of the energy is converted into heat. To avoid overheating, a duty cycle of 20% per hour is often set as the limit. This value can be improved by using ball screws, though their higher initial cost compared to the previously mentioned systems must also be considered.

5. What should you consider when dimensioning lifting systems?

If you’ve chosen an electromechanical lifting system with a screw jack, the next step is to dimension the system:

5.1 Check Static/Dynamic Load Capacity

If your system, for example, needs to support and move a force of 7.5 kN, the MJ2 screw jack, which can handle up to 10 kN, will be sufficient. For dynamic testing, consider the performance limits. An MJ2 can handle up to 0.5 kW at a 20% duty cycle per hour.

5.2 Check buckling force

Depending on the screw length, bearing arrangement, and axial force, the screw may buckle. Longer screws are particularly susceptible to such failures. Buckling safety should be between 3 and 6. Buckling tests are conducted according to Euler or Tetmajer methods.

5.3 Check critical speed for rotating screws

The permissible screw speed also depends on the length. This applies only to the traveling nut version or directly driven screws. Advice: Increasing the screw pitch can reduce the screw speed.

5.4 Calculate the lifespan for ball screws

Unlike trapezoidal screws, ball screws can theoretically be designed for a specific lifespan. Use the manufacturer’s dynamic load rating, along with the axial force and screw speed.

5.5 Check self-locking capability

Since screw jacks are often used in safety-critical applications, self-locking is a significant factor. The degree of self-locking depends on the screw pitch. If the pitch angle is greater than 4.5°, there is no self-locking. At 2.4° to 4.5°, you achieve static self-locking (no movement from a standstill). Below 2.4°, you achieve dynamic self-locking (the system self-brakes).

5.6 Tips and tricks for speed, duty cycle, and more

To increase speed or duty cycle values, consider the following measures:

1. Use oil filling (oil transfers heat better).
2. Adjust the gearing (if the performance is too high, it can be reduced, but this also affects speed).
3. Use ball screws (an efficiency of over 90% allows for higher speeds).
4. Increase the screw pitch (improves efficiency and allows for higher speeds).