Between the shoulder and elbow pitch joints, I see two types of connecting structures on larger robots. I've attached a picture.

enter image description here

My questions:

1) What are each of the respective mechanical parts called?

2) What is their purpose for the robot arm?



3 Answers 3


The "pump-looking" things are either hydraulic cylinders, or mechanical dampers if the robot is electrically driven. EDIT: I'll accept @50k4's identification as hydraulic springs.

In the "what's going on back here" department, the long thin member is a linkage. It is part of a 4-bar parallelogram linkage which allows the forearm to be driven using a motor which is located far below the elbow (the lowest red piece in your picture). The benefit of this arrangement is that the upper arm link does not have to carry the weight of that motor. Please note that I am using "upper arm" in the anthropomorphic sense. It is the link closest to the shoulder.

The other part of "what's going on back here" is the large blob of metal. Notice that this blob has its top center carved out so that the linkage can nest in there, thus increasing the forearm's range of motion. The blob itself is a counterweight. By adding counterweights, the load on the motors due to gravity is reduced, even though the total inertia that the motor must move is larger.


What you marked as "pump looking things" are hydraulic springs. Their purpose is to reduce the load on the motors.

What you marked as "What is going on here" has 2 component.

  1. A mass used as a counterbalance, also to ease the load on the motors and possibly to reduce the radial torque load on the bearing.
  2. The four bar linkage mt allows the move the motor and gearbox of axis 3 alongside axis two, which brings the overall center of gravity to a favourable position (closer to the base).

Both previous answers 'could' be accurate re the "pump looking things" in that they say they are hydraulic springs/dampeners such as you would find on a hatch back rear door, the point of them as mentioned is to reduce the apparent load e.g. if your robots forearm weighs 20Lb and you want to lift a weight of 1Lb the motor would need to apply 21Lb of force but put a 20Lb spring on it and the spring supports the arm and the motor only needs to lift 1Lb. It is also possible they could be hydraulic or pneumatic actuators (hydraulic using oil or water, pneumatic using gas, normally Co2), the liquid or gas is pushed into the cylinder which then forces the rod forward and the rod moves forward at a greater force than the pressure in the cylinder (bore or diameter of the cylinder is greater than the width of the rod so you achieve mechanical advantage - you find these on JCB diggers or tipper trucks. You can see both spring and actuator cylinders in the photo of my robot, the spring is the one holding the body shell open and the flipper actuator can been seen at the bottom of the robot. enter image description here Again both previous answers are correct re the 'Bit at the back' but just to point out linkages may be powered or unpowered, unpowered linkages are simply used to manage axis at the end of an arm section, in the pic below axis A & B will always remain parallel as will C & D. If you were to increase the length of D then A would tilt up away from the B axis as A moves forward. Adding a motor into the linkage can allow finer movement or greater dexterity for the arm. The linkage in the final pic is being used to drive a spike forward and down as the rear flipper goes up. Just to say there's absolutely nothing wrong with previous answers, I just wanted to let you know there are additional possibilities and show some practical applicationsenter image description here enter image description here

  • $\begingroup$ I was not guessing. I know for fact that they are hydraulic springs. The question did not refer to what could these be, but to what are these! $\endgroup$
    – 50k4
    Commented Nov 3, 2016 at 19:32
  • $\begingroup$ My apologies - what did I miss that proves that they are springs rather than pistons? $\endgroup$
    – Lee Wilson
    Commented Nov 6, 2016 at 19:31
  • 1
    $\begingroup$ Number of electrical motors matches number of degrees of freedom, no hydraulic connection on the robot, no valves, no pump. They are pistons, but they work as a hydraulic spring not as motors or dampers. $\endgroup$
    – 50k4
    Commented Nov 6, 2016 at 19:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.