Today’s medical device market demands patient-focused products marked by performance and safety. It’s not enough that systems move on demand. They also need to stop when required, no matter how big the load.

For applications as varied as wheelchair lifts, surgical robots, or radiotherapy machines that move about the patient, safety brakes need to be fast, strong, reliable, and compact. Wrap-spring technology satisfies all these requirements. Wrap-spring brakes generate significant stopping and starting torque in a small package, providing millisecond response times.

From the point of view of an original equipment manufacturer (OEM), these all-mechanical devices are highly customizable. For minimal cost, a brake can be modified to provide specialized performance and device characteristics that make the overall system more effective in the market. Read on to find out how.

The Basics of Wrap Springs

Wrap-spring clutches and brakes use a spring to connect an input shaft with an output shaft and control the relative speed of the two. The basic configuration consists of an input hub (motor side), an output hub (load side), and a helical spring that wraps around them both (see Figure 1). The spring is sized so that its inner diameter is smaller than the outer diameter of the hubs, generating a tight frictional contact. In the most basic implementation, rotation of the input hub tightens the spring so that it wraps down around the input and output hubs to apply a strong normal force. This force accelerates the output hub to match that of the input hub. Stopping or reversing the input hub unwinds the spring to release the output hub.

A wrap-spring clutch/brake uses a spring to connect an input hub (motor side) to an output hub (load side). When the input hub turns, the spring tightens around the output hub (wraps down) to apply torque and accelerate the load. Stopping the input hub unwinds the spring to release the load.

Customizing Wrap Spring Clutches or Brakes

The basic design described above forms an overrunning clutch, used to accelerate the load from a stop and release it upon demand to coast. Depending on the design of the spring and hubs, the device can stop the load at a random position or designated position. Alternatively, wrap-spring technology can be modified to create a stop-start clutch.

Customization #1: Actuation

Wrap-spring brakes are controlled via one or more tangs on the spring. An actuator actuates the tang to cause the spring to loosen or tighten. The actuator introduces multiple degrees of design freedom. OEMs can choose among mechanical, electrical, or even pneumatic actuators, making trade-offs in terms of space claim, cost, and power consumption. Location and interface are additional variables.

Space is always at a premium in medical equipment. If the design includes room in line with the hubs, the actuator can be placed there. If space exists perpendicular to the brake, the actuator can be placed there.

Customization #2: Bidirectionality

A standard wrap-spring brake is unidirectional. Applying the technology in a bidirectional system typically requires a second unit. Although this approach is effective, it typically adds cost and complexity, not to mention size. Alternatively, a device can be customized so that a single wrap-spring brake offers bidirectional performance. Although the modifications result in a device that is somewhat longer than a unidirectional version, the overall system is still more compact than a two-brake version.

Customization #3: Amount of torque

No two medical systems are alike. The torque requirements for a brake on a standard patient bed, for example, are significantly different than the brake requirements for a radiotherapy machine that moves around the patient.

Wrap-spring brakes offer multiple options for tuning the amount of torque without significantly changing the package. The most obvious approach is to increase the cross-section of the spring. This, in turn, expands the surface contact for braking. Just by switching to a heavier duty spring, a wrap-spring brake could increase torque from 30-inch pounds to 50-inch pounds, for example. That constitutes a 60% increase in torque for a minimal increase in diameter.

The spring cross-section can be fine-tuned to balance space and torque considerations. Assuming the change involves a standard gauge of wire, the modification can be done with minimal cost difference and a fast turnaround.

Customization #4: Materials

Different applications impose different requirements on components. A wheelchair lift located outside the building is quite different from the patient bed placed in a climate-controlled treatment room. Here, too, wrap-spring brakes can be customized. Both hubs and springs can be modified to ensure specific properties. Hubs can be machined from bulk material or molded from powdered material that is then sintered to add density.

The material can be selected to address specific environmental conditions. That wheelchair lift exposed to the elements might use stainless-steel hubs, for example. A device designed for a patient room or operating theater would likely use 300-series stainless steel. Powdered-metal hubs have an interesting advantage, which is that the material can be impregnated with oil for self-lubricating properties. The type of oil can be changed depending on the environmental conditions such as temperature, corrosion factors, and more.

Customization #5: Shape

The technology offers a significant amount of flexibility in terms of form factor. Because hubs can be molded or machined, they can be modified for each specific design. This encompasses developing a device that fits within a certain envelope and machining or molding the brake to conform to the contours of the machine, for example to integrate the brake directly into a robotic joint. They also can be built with integrated gears, pulleys, or couplings, and delivered as complete subassemblies.

Customizable Wrap-Spring Brakes From SEPAC

In today’s competitive environment, OEMs want a brake and clutch solution that serves their needs. Wrap-spring brakes offer a unique combination of high-torque, small size, and fast response time, combined with multiple degrees of freedom. By customizing torque, materials, size, and shape, and even responsivity, OEMs can take advantage of the benefits of wrap-spring technology while specifying a device that meets their needs in multiple other ways.

With minimal effort and cost impact, wrap-spring brakes can be adjusted for bidirectionality, torque modification, custom materials to optimize performance, and custom shaping to minimize space claim and more effectively integrate the device into the system.
Powdered-metal hubs have an interesting advantage, in that the material can be impregnated with oil to generate self-lubricating properties. The type of oil can be changed depending on the environmental conditions.

To see how wrap-spring brakes from SEPAC can help improve your device or application, please contact us.