Lifting heavy weights is a great strain in all professions and causes back problems. Especially in nursing which is also accompanied by a lot of bending over. To improve such jobs and to bring relief to nursing staff, mobile patient lifters can be used. A very compact form of drive mechanics suitable for this purpose has been developed by the small drives specialist, Zeitlauf, in a system partnership. The medical field of application and the target market in Europe and North America dictated the stringent peripheral conditions that had to be fulfilled.

Diverse requirements
The first priority in the case of personal transport systems is the safety regulations. These are followed by power, smooth running, easy operation and mobility as well as a long service life. In a medical environment, safety starts with the design; for example, all devices must be easy to clean. The “inner safety” of a lifter corresponds largely to the more familiar specifications of industrial technology. Even if the drive fails, the load must not sag, overload protection and endurance strength of the bearing components must be guaranteed. Besides, in this particular case: being a battery-operated device the mechanics must use the drive energy sparingly; this is the only way to guarantee a long period of use. To this end a lightweight construction and high efficiency is imperative both for motor and gear. A side effect of the lightweight design is that it also improves the handling of the lifter, which has to be pushed by hand.
If one considers that for the targeted sales market patients weighing up to 260 kg have to be lifted and lowered safely, then it is clear that without the contribution of a drive specialist, optimal design is virtually impossible. Apart from the purely technical requirements such devices also have to meet the respective national licensing conditions. In this case, for North America the requirements of United Laboratories (UL) and the Food and Drug Administration (FDA) had to be met. To meet the conditions for release it was necessary to carry out an extensive type test as well as a complex final test under maximum load with detailed documentation as specified by the aforementioned organizations. Here too, Zeitlauf was able to assist by undertaking the tests required to achieve release for mass production.

Technical details
Patients lie in beds or sit in wheelchairs. In order to lift them up, a large variable lifting height is therefore required. However, at the same time the centre of gravity must always be within the footprint of the lifter to prevent it from tipping over. For tasks like this it has been established that the best type of construction is a lifting mast with a bent arm. The long, narrow sheared outriggers will fit under any bed and can therefore support the lifter safely. The batteries are located in the base frame at the bottom of the mast. To accommodate the drive mechanics safely in the mast and at the same time ensure the necessary load-bearing capacity, the only possible means was a ball screw. A positive side-effect is the smooth-running character of a ball screw. The very long, 675-mm spindle axis is prevented from buckling by means of an integrated guide bush. Hence the lifter reaches a lifting height of 1350 mm despite its very small size/volume with the aid of an integrated lift pipe.
The actual function of the drive itself has thus been reduced to a small, compact unit. A highly efficient brushed DC motor  serves as the power source.  The output speed/rpm is reduced by a compact planetary gear with a reduction of 1 : 15.33 in a single stage.  Hence, a motor output of 90 W permits lifting forces of 100 N to 5200 N. In the process the lifter reaches lifting speeds of up to 25 mm/s and can perform 60 complete lifts per battery charge.
In view of the fact that ball screws do not stop of their own accord, a separate braking solution is required. A load-dependent friction brake directly integrated into the gear serves as a safety device. The lifter must be able to hold the patient securely if the motor stops; hence the brake is only activated during the downward movement. A free wheel clutch decouples the brake during lifting, thus saving energy and reducing wear as well as permitting the lifter to be operated for longer per battery charge.  On the one hand the friction brake acts as a safety and holding device when the motor is at stop, and on the other it reduces the lowering speed and thus makes it possible to gently lower the patient. Thanks to the compact drive block consisting of motor and planetary gear with an integrated brake, the entire lifting unit, including the ball screw is very narrow. Despite the small dimensions, the service life of the maintenance-free unit is very high at 4,000 operating cycles.

All pictures by Zeitlauf

Notes for editorial staff: If you require further picture material, for example for a big title photo, please contact Ms. Schmidt. She will be delighted to supply you with a picture that meets your specifications.

Text: Grad.. Chem. Andreas Zeiff, editorial office Stutensee  Grad. Engineer. Dietrich Homburg, editorial office Stutensee

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