The trend in electric actuator design in medical applications is to put as much push as possible in the smallest package. And because the device will be used around patients, it should work as quietly and smoothly as possible.

A number of manufacturers have succeeded well in doing so and on several counts. For instance, one actuator has only a two-in. square and a four-in. stroke. Several develop more than 2,000 lb of thrust. And most come with dozens of options for almost custom applications. Many cylinders have the same form factor of a hydraulic or pneumatic cylinder to facilitate easier replacement. What's more, most units are available with controllers that allow more than constant velocity, although that is the most common operation.

Sizing an electric cylinder calls for running through a few calculations and guidelines. Here's what our experts say.

A few sizing guides

Simple applications such as lift tables are controlled by foot petals or push buttons. But a few others call for an acceleration, a period of constant velocity, and a controlled deceleration — the classic trapezoidal move profile. You need to know a bit about your application to size a unit. For instance, be prepared to find required thrust from:

F_th = F_f + F_a + F_g + F_app

where

F_th = thrust, lb., F_f = friction force, lb.,

F_a = dynamic force of acceleration, F_g = force of gravity or weight, and F_app = any additional applied load, lb.

When an incline is involved in the application, use:

F_th = Wµ cos(θ) + (W/g)(³v/t_a) + W_sin(θ) + F_app

where

W = weight or load, lb.; µ = coefficient of friction; θ = incline angle, degrees; g = gravity, 386.4 ips²; ³v = change in velocity, ips; and t_a = acceleration time.

For example, find the thrust required to accelerate a 200 lb load to 8 ips in 0.2 sec. Calculate this thrust at inclination angles (θ) of 0 and 30°. Assume µ of about 0.15. Include a return mechanism (F_app) that generates 25 lb.

Therefore:

F_{th 0°} = (200)0.15(1) + (200/386.4)(8.0/0.2) + 200(0) + 25

= 30 + 20.73 + 0 + 25

= 75.73 lb

and

F_{th 30°} = (200)0.15(0.866) + (200/386.4)(8.0/0.2) + 200(0.5) + 25

= 26 + 20.73 + 100 + 25

= 171.73 lb.

Of course, the load increases along with the incline to almost 250 lb for a vertical lift. Select the electric cylinder based on the highest load figure. Check with the manufacturer because they may factor in other considerations to their calculations, such as duty cycle and environmental temperatures. Most manufacturers will also assist in the selection.

Another way to size an electric cylinder is online with company supplied sizing software. Take Nook Industries, Cleveland, (nookindustries.com), for example. From the home page, select the actuator with sufficient thrust, and then fill out the other parameters in the fields provided. This will include travel rate and length, sensor options, and mounting options. Essentially, you're customizing the actuator. Designers can then download a 2D drawing or 3D model of the actuator that can be included in the CAD model of the mechanism.

Other companies here will do similar sizing feats, but over the phone. One, Joyce/Dayton, provides sizing software for their industrial jacks.

A few applications:

“One unusual application,” says John Walker, chief engineer with Exlar Corp., Chanhassen, Minn., (exlar.com), “assists with the injection of a viscous high-contrast liquid that make tumors more visible in X-rays. Standard procedure was to hand inject the liquid from a large syringe. But the load was so great, medical assistants quickly tired and could not keep the fluid moving at the needed constant rate. The solution was to let an Exlar cylinder drive the syringe at a constant speed. Automated syringes are now used to inject several viscous fluids.

A similar application generates relatively high-pressure water to wash debris out of veins. The catheter with a cleaning tool would be pulled out of a vein while high pressure water, generated by the automated syringe, would wash out particles or deposits that worked loose.

Nomenclature and Conversions

V_max = maximum velocity, in/sec

V_ave = average velocity, in./sec

t_a = acceleration time, sec.

t_d = deceleration time, sec.

t_cv = time at constant velocity, sec.

t_tot = total move time, sec.

1 N = 0.2 lb

1 lb = 4.4 N

Tiny actuator pushes with 4.5 kg.

A Size 8 hybrid external linear actuator, from Haydon Switch & Instrument, Waterbury, Conn., (hsi-inc.com) is only 21-mm square, making it the world's smallest, according to its manufacturer. External refers to the lead screw which does not retract into the body of the actuator. As the motor steps, the lead screw rotates, and only the mating nut translates. The hybrid reference means the motor is a cross between a variable reluctance and a permanent-magnet stepper design. This combination is said to make for a more robust, accurate, and longer life linear actuator.

The actuator replaces four different components: a motor, coupling, lead screw, and nut. The design does not need a shaft-to-lead-screw coupler because the rotating Acme stainless-steel lead screw is incorporated into the motor's rotor.

The actuator comes in a range of resolutions from 0.0015 to 0.04-mm/step and it delivers 4.5 kg thrust without compromising long life or cost, say developers. The lead screw end can be machined to use an out-board bearing support for long lead screw lengths.

Actuators sport a linear motor

PowerRod actuators and cylinders from Parker Hannifin, Europe, (parker.com) provide several units with 51 to 102 N thrust and speeds to 5.9 m/s. Strokes range from 27 to 309 mm.

The design has a IP67-rated forcer, stainless steel thrust rod, and rare-earth magnets to produce peak forces to 780 N. Repeatability is six microns while accuracy from the integral non-contact position sensor is 250 mm. The reliability of a linear motor makes them almost maintenance free, and because their operation is essentially ‘non-contact’, they do not wear, so they maintain their precision over their lifetime. The internal dry bearing also ensures quiet performance.

The design caters to high duty cycles without forced air or water cooling, as the tubular motor uniformly radiates heat.

Actuators push 500 lb to 18 in.

Linear actuators from Joyce/Dayton Corp., Dayton, Ohio (joycedayton.com) come in machine and ball-screw designs. Standard travels are 3, 6, 12, and 18 in., and have a double-lead machine screw. The heavy-duty actuators can handle loads to 2,000 lb. The company says it tests the design at full capacity for thousands of cycles. The lifting mechanism has a high-tensile aluminum-bronze lifting nut or heat-treated ball nut connected to a heavy wall stainless-steel-lifting tube. Actuator housings are cast from rugged yet lightweight aluminum and include a cast mounting clevis.

The enclosed 115 Vac motor uses a permanent split capacitor. Load/no-load speeds are about equal. A thermal overload device opens and resets as needed.

The enclosed 12 Vdc motor tolerates the weather an uses permanent magnets. These act as a secondary brake for added safety. The motor is smaller, cooler running, more efficient, and has higher duty cycles than series-wound motors. Lower current draw provides for longer battery life. It's also equipped with a thermal-overload device. The rotation is reversible by reversing the two color-coded leads, and torque is the same in either direction.