Mechanism Needed for a Compact, High-Torque Transradial Prosthetic Arm

[KavehSanaei]

Hello everyone,

I'm collaborating with a group of engineering friends to develop a transradial prosthetic arm that uses a mechanical claw for gripping. This prosthetic is designed for physical activities, demanding both strength and precision. We're aiming for a pinching force of around 8 Nm (~70 lb-in) and are using a low-torque, high-speed motor (e.g., ElectroCraft RPX22) combined with a 100:1 gearbox.

Our challenge is maintaining this torque during the gripping action, which could last from several seconds up to a few minutes. The mechanism must ensure zero backlash during pinching for safe use.

We're exploring various mechanisms to hold the claws in place effectively:

• Cam mechanisms
• Zero-degree ratchet systems for zero backlash
• Locking via hydraulic or pneumatic pistons
• Utilizing the intrinsic torque of the motor and gearbox

Additionally, the mechanism needs to be compact and lightweight since it is a prosthetic.

We're looking for suggestions. What mechanism would you recommend for maintaining the claw's position under torque in a compact and lightweight design? We're open to ideas, experiences, and any suggestions you might have.

Thank you in advance for your input and ideas!

 

[berkeman]

Welcome to PF.

to develop a transradial prosthetic arm that uses a mechanical claw for gripping.

I'm not familiar with the term "transradial" in the context of a prosthetic arm. Are you referring to the radius bone, or is there some other meaning? Thanks.

And can you upload some drawings of your concepts so far? Use the "Attach files" link below the Edit window to upload a PDF or JPEG image of your work

 

[KavehSanaei]

Below I have attached a rough drawing of what we are trying to design. Both sides of the claw need to move independently. Two motors move each side of the claw independently. The gearboxes drive a cable and pully system that articulates the claws. We want the calws to have zero backlash once they are in position.

I also included an example picture of a transradial (below the elbow) prosthetic. It being transradial is not relevant to the mechanism itself.

 

[berkeman]

Both sides of the claw need to move independently. Two motors move each side of the claw independently. The gearboxes drive a cable and pully system that articulates the claws. We want the calws to have zero backlash once they are in position.

How are you going to achieve zero backlash in this? Can you post a more detailed drawing?

 

[Tom.G]

Instead of Zero backlash, which is rather difficult in the real world, I suggest a rather hard compliant material covering the jaws/grippers -- maybe a semi-hard polyurethane. Polyurethane came to mind because it is not attacked by most chemicals and can be formulated and cast in about any hardness and shape you want.

This could be chosen for maybe a millimeter of compression at maximum force.

[edit/afterthought] If you Really require Zero Backlash, use a spring on the grippers to hold the grippers closed and use the motor to open them.

Cheers,
Tom

p.s. please keep us updated on your progress/solutions, we like to learn too!