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Knee design for a heavy duty robot Question
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Posted by: robotdesigner

06/25/2004, 12:24:15

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I'm trying to design a robot which is similar to that of a human leg. The leg of the robot must be able to lift at least a ton and the knee of the robot must be able to bend to the same range than that of a human leg. Unfortunately I don't have much experience in mecanical engineering.

I tried to design the knee by joining the upper leg and lower leg at a common axis of rotation through an axel. Unfortunatly, after a small amount of bending both legs collide into each other limiting the range of rotation of the leg. It is possible to modify one or both parts of the leg to enable a bit more range of rotation, but this makes the structure of the leg weaker.

I thought of another design that would enable a very large range of rotation. I modeled the knee of the robot in a similar way than that of a human knee. In a human knee, one bone is in contact with the other and rotates around it. The bones are held together through ligaments. Similarly, the common extremity of the upper leg and lower leg of the robot are made as a gear (each gear is spanning 180 degrees only). The gears are maintained at a very close but fixed distance from each other through a metal frame (in the center of each gear, there is an axel where the frame is attached. Hence there are two axels in this design while in the design there was only one). A cable is connected at the front and a second cable is connected to the back of the leg (on the upper and lower leg)would enable the leg to be bent. In such a way the leg could bend as much as needed.

However, I have many questions on the subject. In such a configuration, does the gears need to be lubricated? Is lubrication crucial? How much would the lifespan of the gears would be reduced if I don't lubricate?

In practice, I haven't seen this latter design anywhere. It seems robust and very flexible. Is there any important reason why this design is not good? Is this design too complex? Is it flawed? Too costly? Would there be too much stress applied on the gears?

Any information on any of those questions would be greately appreciated.

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Knee design
Re: Knee design for a heavy duty robot -- robotdesigner Post Reply Top of thread Forum
Posted by: RandyKimball
06/25/2004, 19:21:05

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It is hard for us to see your knee design as a word discription "seeing" that we can not see into your head. ... wink ... However, I'll try to express some of the answers as I best understand the questions. The problem with asking mechanical engineering questions is ... they need to often be expressed in mechanical engineering language.... Let's see if we can wade past that.

As for the lubrication portion; many gears are designed of materials that lube themselves, don't need lubed, are lubed by a timer,or use an open gear grease ... others. Open gear grease is similar to tar in texture (and so is the mess it makes). If you need to lubricate depends on the load, speed of action, materials, and gear lash. My GUESS is you could do with a (clean) dry silicone lubricant just fine, but I suggest you keep an eye on the gear wear pattern for an R&D length of time. I do not suggest a graphite as some may suggest, graphite is more abrasive than many realize. A lubricant has two main functions, first to reduce friction and wear, and second to reduce heat (all related). These actions you can observe during R&D and react accordingly IF necessary.

I think your method is used on some assembly and welding robots, if I understand it correctly.

More common for devices pivoting with heavy loads the joints like used in back-hoes are used. The "muscle" is with hydraulic rams or ball screws. Ball screws are very effecient and recently have become very affordable. It doesn't take much torque to turn the ball screw and it delivers high mass moving forces and accuracy of position. If turned by a step motor the position can be calculated to repeat well. (If I were doing this project, I would use ball screws & ball screw nuts and step motors.) Also, step motors provide the best torque at the best size and money. The newest generation of step motor drives are very intelligent and quite inexpensive. Many will provide the required phased steps to the motor after being provided with no more than a distance instruction expressed in terms of rotations down to even micro-portions of rotations. .... very easy on the engineer!

Have I helped?

The worst suggestion of you lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

Modified by RandyKimball at Fri, Jun 25, 2004, 19:29:21

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