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Introduction

Design of a Controlled Locking/Unlocking

Wrap Spring Clutch Knee Joint

Project Engineers: Derek Carr, Kevin Smyth, Dillon Younge

Project Co-Advisors: Dr. Anne Robertson, Dr. Nitin Sharma, Nicholas Kirsch

Project Objective

Design Testing

AcknowledgementsMr. Robert Powner, Ace Wire Spring & Form Company

Mr. Thorin Tobiassen, Swanson School of Engineering Machine Shop

Mr. Andrew Holmes, Swanson School of Engineering Machine Shop

Dr. Stephen Ludwick, Aerotech, Inc.

Bibliography

Discussion and Conclusions

Creating a system which would allow paraplegics to walk for a brief period would improve

their cardiovascular health, increase bone density, and stimulate their muscles. Such a

system would benefit millions of paraplegics worldwide. Many researchers, including those

here at the University of Pittsburgh, are trying to create such a system. In other labs, either

Functional Electrical Stimulation (FES) or fully-powered orthosis systems have been

designed to achieve this. What makes our project unique is that it combines the use of a

motor for hip extension and flexion and FES to control extension and flexion at the knee.

However, the use of FES quickly leads to muscle fatigue. Our solution to minimize muscle

fatigue caused by FES during the gait cycle was to create a controllable locking/unlocking

knee joint. This knee joint will unlock to allow the knee to bend during the swing phase of

the gait cycle, but be able to lock to support the weight of a person during the stance phase.

While FES will still be used to extend the leg during the swing phase, it will no longer be

needed to “lock” the knee during the stance phase.

Figure 1: Gait Cycle with Clutch Cond it ion  

[1] Irby, Steven E., et al. "Optimization and Application of a Wrap-Spring." IEEE Transactions on

Rehabilitation Engineering June 1999: 130-134.

[2] Winter, David A. Biomechanics and Motor Control of Human Movement. Fourth. Hoboken: John Wiley &

Sons, Inc., 2009.

Overall Project:

The mechanism for the walking orthosis must meet the following criteria during a gait cycle:

• Lock during standing to prevent knee buckling

• Unlock at the start of the swing phase to allow both knee flexion and extension

• Only allow knee extension and prevent hip flexion at the end of the swing phase

• Lock at the end of swing phase and beginning of stance phase

• Allow a controllable knee flexion during the stance phase

Our Scope:

Our specific objective was to design a one way wrap-spring clutch mechanism to lock and

unlock the knee joint during walking and standing via a linear actuator. The mechanismmust support the weight of a person to provide stability during standing or in the stance

phase. 

The wrap spring clutch knee joint we designed will be incorporated into the complete

walking orthosis in Dr. Sharma’s lab. By locking during the stance phase and

supporting the weight of the user, the knee-joint will minimize the use of FES, thereby

limiting muscle fatigue. Our design is lighter and slimmer in profile than commercially

available wrap spring clutches. Future work to be completed on this design includes

incorporating the control system to lock/unlock the clutch via the linear actuator at

specific intervals in the gait cycle.

When both the machining and assembly of the clutch were completed, the holding

torque (gripping torque) was tested. A spring scale was used to determine the

maximum torque that the clutch could handle. Based on the required torque of ~54 N-m

mentioned earlier, the clutch had to hold a minimum of ~40 lbs at a distance of one foot

from the pivot point. To test for this, the thigh bracket was clamped in a vice while the

calf bracket had a load of 40 lbs applied (at 12 inches from the center) and measured

with the spring scale.

Based on our research, we discovered that the knee joint must withstand a minimum

of 0.6 N-m of torque for every kilogram of body mass [2]. Thus, for a 200 lb person,

the required torque was determined to be ~54 N-m.

Design Components :  

1. Thigh Bracket   – Connects to upper thigh portion of the existing frame. (actuator support isbolted on)

2. Inner Cylinder   –Pressed and welded to thigh bracket with ease-of-weld flange design.3. Linear Actuator   – Controls wrapped condition of Wrap Spring by actuation of the Control

Collar. Maximum actuation force of 24N with 20mm stroke.

4. Flanged Bushing   – Separates the Brackets and prevents binding from metal on metalcontact. Delrin resin polymer

5. Calf Arbor Hub  – Pressed and welded to Calf Bracket with ease-of-weld flange design.

6. Calf Bracket   – Connects to lower calf portion of the existing frame (near ankle).7. Bushing  – Prevents friction and binding between the two, aluminum Arbor Hubs. Delrin

resin polymer

8. Wrap Spring   – Creates torque through interference with Arbor Hubs. Maximum torque of112 N*m. 4 turns with 0.01 mm interference. End tangs fit into Control Collar and Thigh

 Arbor Hub.

9. Control Collar   – Connects the control tang of the Wrap Spring to the Linear Actuator. SLA3D Printed.

10. Thigh Arbor Hub  – Connects to Thigh Bracket by way of hex fit on Inner Cylinder. Fixes thestationary tang of the Wrap Spring.

11. E-Clip  – Holds clutch components together.

Where, =

=

2   = ℎ

  =

2   =

=

=  

For the gripping direction, [1] 

= 22 −

22

2 − 1  

For the overrunning direction,

0 = 22 −

22 1 −

2  

8

10

5

7

4

6

9

3

2

1

11

The equations used to determine the gripping and overrunning torques

of the wrap spring clutch are shown below. As seen in these

equations, the number of active coils of the torsion spring and the

coefficient of friction between the spring and the arbors exponentially

influence the torques.