Lower limb prosthetics have been around for more than 200 years. But, it’s only in the last 40 years or so that they’ve progressed to the point of accurately mimicking the human body.
The ankle joint, in particular, has been hard for biophysicists and mechanical engineers to copy. Hugh Herr, however, hopes to change all that.
Herr heads the biomechatronics group at the Massachusetts Institute of Technology Media Lab, and is a double amputee. He has also been called the Leader of the Bionic Age by TIME magazine.
In 2012, he developed the BiOM ankle system, which is the only lower leg prosthetic with an active ankle joint that performs plantar flexion—also known as step off movement. The system has been clinically shown to power an amputee forward so he or she walks with normal levels of speed and metabolism.
The system has evolved in recent years, and the third-generation version is now called Empower and made by the German prosthetics company Ottobock. A key part of the development process for the BiOM system involved finding the best lubricant for a small precision ballscrew within the prosthetic. This ballscrew is actuated by a battery-powered electric motor and continually adjusts the angle of the prosthetic foot relative to the lower leg.
Product designers contacted Nye Lubricants Inc. in April 2013 to discuss the lubricant issue. The two parties then worked together over the next 14 months to find the lubricant that would maximize ballscrew life and minimize battery size.
Nye engineers developed a test system to try various greases with the ballscrew components at the range of speeds and forces that the ballscrew undergoes during prosthetic use. Initially, 2-millimeter-diameter steel balls were to be tested for 2 hours, but they experienced seizure after only 9 minutes. The second test involved the use of ceramic balls and lasted the full 2 hours.
Nye engineers then tested its Rheolube 374A grease, which has a polyalphaolefin oil base and a lithium soap thickener. The highly fortified grease offers excellent corrosion resistance, performs well under high shock loads and has an operating temperature of -54 to 177 C. As a result, it is often specified to lubricate the landing gears of wide-body aircraft.
Engineers first tested the grease with ceramic balls, but it failed after only 43 minutes. However, the grease lasted the full 2 hours when tested with steel balls.
A 16-hour retest was then done, and 374A consistently performed well for the entire length of the test. The grease also withstood a step load force of 2,000 newtons, per ASTM D-5706.
In addition, it performed well in a standard four ball wear test, which involves rotating a steel ball against three lubricated stationary steel balls under a specified load, temperature, speed and time, per ASTM D-4172. In that test, the grease allowed a wear scar of only 0.44 millimeter on the three stationary balls when tested for 1 hour at 1,200 rpm.
The positive results of all these tests convinced the prosthetic manufacturer to use steel balls in its final ballscrew design and Rheolube 374A in the ankle prosthetic. For more information on lubricants, call 508-996-6721 or visit www.nyelubricants.com.
