Design Requirements
To design a rope gripping device that can be mounted on the hand of a C7 Quadriplegic athlete fulfilling the following:
- Capable of gripping sailing lines of varying sizes from 3/8in to 1/2in with a pulling force of 50 pounds.
- Pulling force of the rope is to be transmitted to the wrist and not the fingers.
- The signal input can come from small movements of the thumb.
- Compact and lightweight device that is easy to remove and put on even during sailing without assistance.
- Durable and water resistant since it is to be used in a wet and salty environment.
- Ability to be continuously used for a minimum of 4 hours of sailing, preferably for a full day without recharging.
- Ability to grab the rope fairly quickly and release it instantly if needed.
- Have as few hooks or sharp edges that would accidentally snag a sailing line.
Theory
The main kinematic principle behind the device developed is the toggle position of a four bar linkage. The mechanism exploited the properties of a toggle point similar to an industrial four bar clamp shown in Figure 1. In these devices, three of the four pin joints become aligned when moving the handle of the clamp to the closed position. This causes the jaws of the clamp to shut on an object and not release due to the high mechanical advantage (MA) at this location.
Figure 1: Four bar clamp in closed position with three pins aligned
The mechanical advantage can be understood from the following equation:
where r represents a radius from the input or output force relative to the instant center of rotation and ω represents the angular velocity of the input or output link. As the input link (handlebar) rotates the fourbar into a toggle position, the output link's angular velocity tends to zero. According to the above equation, this causes the mechanical advantage to tend toward infinity at this point. This enable the joint to "lock" once movement slightly beyond the toggle point is reached. This feature was utilized for holding and gripping the rope.
Another mechanism that the developed device utilizes is the cam cleat. This is the same concept sail boats already use to hold the ropes on the boat itself. If a rope is pulled in tension one direction as seen in Figure 2, the internal torsional springs of the cam cleat create a compression force to grip the rope. As more tension is applied to the rope, the torsional springs are able to apply more gripping force to the rope itself. This enables powerful rope holding ability without the need for external, powered force generation.
Figure 2: Cam cleat mechanism grips ropes in tension
Image Sources: http://www.directindustry.com/prod/emile-maurin/product-39388-754471.html , http://www.cruisersforum.com/forums/f116/best-way-to-re-attach-cam-cleat-to-mast-107621.html