Nordborg, Denmark —In the past, care bed manufacturers had to rely on linear actuators and a complex system of lever arms to produce the rotary motion required to raise and lower the bed's head and foot sections. Now Linak engineers have developed what they say is a simpler, quieter, and economical alternative: The RA40 rotary actuator consists of a 24V dc permanent magnet motor that turns a series of gear stages to produce a working torque output of 184 ft-lbs (350 Nm) via a rotating arm.
"Quite often the motor in these linear actuators is sitting at a 90° angle, which is not very convenient," says Research and Development Manager John Frost. "So we set out to develop a rotary actuator to fit into a compact, 2.76 × 2.76 inch (70 × 70 mm) tube that could be integrated directly into the bed frame."
Two other boundary conditions—a maximum noise level of 45 decibels and desire to use a standard dc motor with a torque output of just 0.15 ft-lbs (0.2 Nm)—imposed additional constraints.
The biggest challenge for engineers was figuring out a way to get the necessary speed reductions with gear stages that all had to be in-line with the motor. "Otherwise, we could have used an ordinary worm gear, which obtains a large speed reduction but transmits power through right angles," says Frost. Likewise, engineers initially tried to use a high-efficiency planetary gear for the first stage, but the noise output at the input speed of 2,400 rpm was unacceptable.
Engineers ultimately achieved the desired torque through a three-stage gearing system (see table next page). The first stage consists of a combination of two worm gears and a crown gear. In order to keep costs down, the gears are made of injection-molded nylon, which has the extra advantage of eliminating any need for bearings. To ensure that the two nylon worm gears would be able to transmit the necessary torque, says Frost, engineers designed them to work in parallel, each picking up half the load.
Linak's RA40 rotary actuator fits into an aluminum or steel tube that integrates directly into a bed frame. As the shaft turns, a lever arm pivots about its center point, rotating the head section of the bed up to 85 degrees and the foot section up to 55 degrees. Using a standard 24V dc motor with a torque output of a mere 0.2 Nm, engineers increased the torque significantly (to 350 Nm) through a 3-stage gearing system (international patent application pending).
The middle stage is a straightforward planetary gear, also made of injection-molded nylon. Although the reduction rate is only 3.7, this stage is needed in order to transmit a sufficient torque to the last gear stage.
Design of the third stage proved especially problematic, since engineers still needed to achieve a substantial speed reduction rate, but they did not want to add cost to the design. "You can always make a gear out of milled steel, but that will get expensive," says Frost.
The solution was a modified planetary gear made out of sintered metal, cut into two halves, press-fit into a nylon housing, and attached at the center point from which the shaft arm extends. Four planet pinions run on steel shafts with plastic bushings. "With an ordinary planetary gear, we were getting a bending moment due to the high torque," recalls Frost. "By splitting the outer ring of the gear into two rings, we were able to balance the forces and meet the 10,000 life-cycle rating."
The fact that this planetary gear's efficiency is only 46% was actually an advantage in this application. "We would have preferred a much higher number, of course. But when a gear's efficiency is lower than 50%, it is self-locking to the torque, which means it cannot be back-driven, and the bed will remain in position," explains Frost.
When the end of the rotational angle is reached, a lever attached to the last stage of the gear hits a microswitch, signaling the control system to cut power to the motor. Don Knotts, who could not keep a camp cot from repeatedly folding up on himself in a classic comedy routine, would certainly have appreciated this feature!
Contact Derek Manz, Linak U.S., 2200 Stanley Gault Parkway, Louisville, KY 40223 ; Tel: (502)253-5595; Fax: (502)253-5596; www.linak-us.com; or Enter 501
|Increasing torque, reducing speed, step-by-step
||24V dc motor
||Modified planetary gear
|Speed (rpm) Max load
|Speed (rpm) No load
||1 : 33
||1 : 3.73
||1 : 53.2
||Injection molded nylon
||Injection molded nylon