Functions and performance of Lifting System Components

What you should consider when designing a screw jack system:

1. System assembly

A screw jack system consists of one or more lifting elements, connected via shafts, couplings, and possibly distribution gears, along with a motor. A typical application is adjustable theater stages, where the stage floor can be raised or lowered, or exchanged for a different stage floor. The assembly of the lifting system can be designed freely within the bounds of technical feasibility. Here’s an example of a lifting system:

2. Connecting elements for torque transmission

Connecting elements are used to link the drive and output shafts of screw jacks, bevel gearboxes, and motors. They come in various designs, each suited to different application needs. Below, we cover the most commonly used types.

2.1 Torsionally elastic couplings

Torsionally elastic couplings are mechanical connections between two shafts that transmit torque and motion from one shaft to another. They are primarily used to transfer torque smoothly from a driving source, such as a motor, to a driven component, like a pump or shaft. These couplings are designed to absorb normal torque fluctuations and vibrations while maintaining precise rotation. They are employed in a range of applications where smooth and reliable torque transmission is essential.

In torsionally elastic couplings, torque and rotational motion are transmitted through a form-fitting mechanism. The connection is made via coupling claws, with an elastic spacer ring used to dampen torque shocks.

The spacer ring also functions to compensate for angular misalignments or axial offsets between the connected shafts. By using different materials for the intermediate rings, the permissible torque values and the resulting torsional angle within the coupling size can be influenced. Materials for the couplings themselves include standard aluminum alloy, as well as gray cast iron, steel, or stainless steel.

The dimension of torsionally elastic couplings follows the permissible torque values specified in our catalogs and the maximum possible angular or axial misalignments, considering an application and operating factor. These factors depend on the type and intensity of acceleration, the number of attempts, and the prevailing ambient temperature. For simplification, under normal conditions without extraordinary operating conditions, a general operating factor of 2 can be used to determine the required coupling size. The operating temperature range of the couplings is from -40°C to +90°C (-40°F to +194°F), with a short-term maximum of 120°C (248°F).

3. Torsionally elastic overload couplings

To protect lifting systems with an electromechanical drive from overload or failure in special applications, the use of a torsionally elastic overload coupling is recommended. This coupling decouples the drive from the subsequent drive train in the event of a sudden blockage. In practice, it has been shown that simple overload couplings, based on the principle of frictional engagement, often fail to provide reliable monitoring. This is due to friction linings, which can stick or corrode over time, leading to unreliable function in the event of an overload.

Furthermore, uncontrolled slipping can disrupt operations and potentially cause greater issues due to excessive heat generation.

Therefore, overload couplings with a slip-free and static friction-free design, combined with electrical disconnection, offer significantly better protection against unintended blockages and overloads. The torque to be transmitted is set via hardened coupling discs with balls as power transmission elements, keeping the slip torque nearly constant throughout the coupling's lifespan. This design, when paired with electrical disconnection via a limit switch, ensures high functional and operational safety. In the event of a shutdown due to operational disturbance, the coupling automatically re-engages and resets to normal operation.

Overload situations can also be monitored and prevented through the control system. An electronic load monitor can determine the effective power consumption of the drive motor and shut down the lifting system in case of overload or higher load.

4. Cardan shafts GX

Highly elastic cardan shafts have proven particularly effective for transmitting torques over greater distances between screw jacks. These cardan shafts evolved from an earlier solution involving simple shaft-coupling-support-bearing assemblies. The cardan shaft series was developed in two variants to meet the demands of lifting systems: one for speeds up to nmax = 1500 min-1 (GX) and another for speeds up to nmax = 3000 min-1 (GX-Z). These shafts meet all necessary criteria, including torsional stiffness, low vibration, damping, and the ability to accommodate angular misalignments. For certain shaft lengths and speeds, support bearings are not required. For large distances between the screw jacks and bevel gearboxes, intermediate support for the cardan shafts can be provided, as the diameters of the thin-walled cardan shaft tubes are matched to standard support bearing sizes.

The dimensioning of the cardan shaft depends on the torque to be transmitted, as well as on the speed and length. The critical bending speed is used to determine the installation length of the shaft.

The following general formula is used for verifying the transmitted torque:

When dimensioning for the transmitted torque, note that only the shafts themselves are calculated. However, the maximum values of the couplings at the ends of the shafts must still be considered.

The dimensioning of the cardan shafts was done according to the standard machine engineering value of the twist angle of ¼° per meter of length.

Depending on the type of cardan shaft, "GX" and "GX-Z," operation is possible at ambient temperatures from –20°C to +150°C ("GX-Z" max. 80°C) (–4°F to +302°F ("GX-Z" max. 176°F)), with the transmitted torque value significantly decreasing at temperatures above 80°C (176°F).

5. Cardan shafts VR

To provide a more cost-effective solution for simpler applications and tasks in linear drive technology, a series of cardan shafts has been developed. These shafts meet the requirements for torque stiffness and damping, with slightly reduced angular misalignments, and are cheaper to manufacture and easier to assemble.

This series is designated as the VR cardan shaft and is usable up to a maximum speed of 1,500 RPM and 245 Nm of torque. The operating temperature range is from – 40 bis zu + 90°C (–40°F to +194°F), with a short-term maximum of 120°C (248°F).

Compared to the elastic cardan shaft "ZR" and the highly elastic series "GX" and "GX-Z," there is a significant difference in terms of installation and flexibility. The "VR" series features clamping hubs that transmit torque via friction and are additionally secured with a locking screw after height adjustment of the screw jacks. Moreover, cardan shafts can be used more flexibly since the coupling hubs are not limited to a form-fitting connection through keyway grooves.

6. Lifting system performance

The performance requirements of the system, including the necessary speed, torque, and motor power, are crucial for selecting the right screw jack components. These parameters should be aligned with the technical specifications of the screw jacks.

7. Torque

The required torque depends on the load that the lifting element needs to move or lift. Consider both the maximum and minimum torque required for the application and ensure that the motor and gearbox can provide sufficient torque.

7.1 Determining the required torque

7.2 Calculating power consumption

7.3 Calculating acceleration power

In high-dynamic drives with high lifting speeds (e.g., v > 10 m/min) and high acceleration values, it is necessary to add the acceleration power Pa to the power required for moving the load to determine the motor power. However, this situation is relatively rare in screw jacks.

8. Transmission gearing

Transmission gearings are used to adjust the speed and torque between the motor and the screw jack. Choosing the right transmission gearing is crucial to achieving the desired performance and accuracy.

The gear ratio value is a key factor in calculating lift per revolution, lift speed, ball screw lifespan, drive speed (spindle), and torque per gearbox.

You can find the appropriate gear ratio for your desired component in our catalog.

9. Accessories

The right accessories enable both designers and users to optimally adapt our screw jacks, spindle screw jacks, electric cylinders, actuators, screw drives, bevel gearboxes, and linear chains to specific installation situations. With these accessories, it is even possible to design, assemble, and synchronize complete lifting systems.

For this reason, we have developed a wide range of accessories to meet your needs. All accessories are available in the same high quality as our entire product range. Besides our extensive selection of standard accessories, we can also accommodate individual customer requests and develop customized solutions. Our dedicated engineers are available for consultations.

A detailed overview of all accessories can be found in our catalogs.

9.1 Typical Accessories

1. Torsionally elastic cardan shafts
2. Cardan shafts
3. Connection shafts
4. Support bearings
5. Motor bells for motor mounting
6. Three-phase standard motors
7. Torsionally elastic couplings
8. Limit switches
9. Handwheels
10. Position indicators

9.2 Bevel gearboxes – Essential connection in a lifting system

A bevel gearbox is used whenever a sophisticated lifting system needs to be operated with a single motor. The bevel gearbox can redirect torque and distribute it to multiple lifting drives. This offers several advantages:

Advantages of the Bevel Gearbox:

✔ Economical alternative for coordinating multiple lifting drives in a system

✔ Transmission ratios from 1:1 to 6:1 available

✔ High efficiency with efficiencies over 96%, resulting in minimal power loss

✔ Various drive shaft designs allow for versatile configuration options for systems

There are 8 different sizes available, from V065 to V260, with the following features:

1. VL version with flange for direct motor mounting according to IEC standard
2. Maximum output torque of up to Tmax = 2300 Nm
3. Duty cycle of up to 100% possible
4. Suitable for use in various environments, including food processing, outdoor installations, and explosive areas

This makes the bevel gearbox an extremely flexible and powerful solution for complex lifting systems that aim to use only one motor.