ENGINEERING INSIGHT
 |
A patient hoist from Arjo that makes use of a Zeitlauf Gearmotors actuator can accommodate loads of up to 260 kg.
|
Patient hoists are an essential adjunct in modern healthcare, providing protection against injury for patients and staff alike. But to assure their safety, the hoists must meet a variety of strict requirements. For instance, regulations from CEN in Europe and FDA in the United States prescribe strength and endurance requirements, effective overload protection, and assurance that the hoist will not sag or drop if the drive fails. In addition, hoists that are battery powered must be sufficiently efficient to maximize the number of lift and lower cycles between battery charges.
In addition to the challenge of meeting regulatory requirements, healthcare equipment specialist Arjo International AB (Eslöv, Sweden) needed to design a lift that could accommodate a range of lifting heights, making it suitable for patients lying in bed or sitting in a wheelchair. The hoist also had to support loads up to 260 kg and be compact and lightweight enough to facilitate mobility. To address these needs, Arjo turned to motor and gearbox expert Zeitlauf Gearmotors Ltd. (Buckingham, Bucks, UK), which has experience in the healthcare sector and in-house product development facilities.
Lightening the Load
 |
Arjo tests the hoists’ actuators individually at 150% of full load prior to shipment.
|
Design engineers at Zeitlauf Gearmotors concluded that the need for compact construction along with a high load-bearing capacity and lifting range necessitated the use of a ball-screw drive that would provide the additional benefit of smooth operation. “A ball screw has a much better efficiency than, say, a trapeze screw,” says Zeitlauf design engineer Matthias Albrecht. The result is a low current demand, which supports a long battery lifespan, he explains.
Ultimately, a drive was selected for the hoist with an 800-mm spindle axis and integral guide bush to prevent buckling under load. This allows the lifter to reach a height of 1350 mm despite its small dimensions. Zeitlauf chose a small but efficient brush-type dc motor used in conjunction with a planetary gearbox that provides a reduction ratio of 1:15.33 in a single stage. This arrangement allows the 90-W rated motor to produce lifting forces ranging from 100 to 5200 N, while the hoist achieves operating speeds of up to 25 mm/sec.
Since ball screws are not self-braking, Zeitlauf built a load-dependent friction brake into the gearbox. This is actuated during downward motion, enabling the hoist to hold the patient securely, even if the motor stops. “When the patient is lowered, the brake system begins operating and the current limits are set from 2 to 7 A,” Albrecht says. A freewheel mechanism decouples the brake during lifting. This reduces wear, but also saves energy by reducing the load on the motor. This mechanism and the inherently high efficiency of the motor and gearbox assembly enable the hoist to perform at least 60 complete lifts per battery charge.
The final implementation of the drive block, comprising the motor, the planetary gear, and the integrated brake is compact and light, yet has been shown through exhaustive testing to offer an expected service life exceeding 40,000 operation cycles under full load. This compares favourably with the 10,000-cycle requirement laid down by CEN, and the 20,000-cycle requirement that had formed part of Arjo’s original brief.
After prototypes had been type tested in line with CEN and US FDA regulations, the Maxi Move patient lifters entered production. To ensure that every drive supplied for the production models satisfies the stipulated performance criteria, Zeitlauf tests the units individually at 150% of full load prior to shipment, using a specially developed dedicated rig. “The test rig that we manufactured uses a pressure cylinder to apply load,” Albrecht explains. During the lift and lower cycle, the tool measures the speed, current consumption, and concentricity of the ball screw under full load. It is rejected if it fails to meet the 2–7-A current-limit requirement. “A computer records all of the data,” he adds. “Every test graph is saved under a serial number that is then issued to the actuator for future reference, if required.”
