Why add balance cylinder to J2 axis of robot? 99% of mechanical engineers choose to ignore it.
Just studied Japanese and German robots in the workshop, and found that slightly larger robots, J2 will be equipped with a balance cylinder, Japanese FANUC robot uses spring type balancing, German KUKA uses hydraulic type (diaphragm accumulator) or counterweight block method, I personally think that hydraulic (diaphragm accumulator) way is the best, the output is basically constant, and it occupies little space.
Why use a balance cylinder for the robot J2 axis?
Let's take a look at the actual situation, the following figure is the nameplate on the FANUC R-2000iC/125L robot 1/2/3 axis motor, the motor power is 4.3KW, and the robot 4/5/6 axis motor is 2KW.
Careful students must have found that the motor power of the original robot 1/2/3 axis is the same, have you ever thought about why it is the same?
Look at the 6-axis articulated robot, in the case of the same load at the end, the torque generated by the load on the J2 axis is definitely much greater than the J3 axis, through the robot arm span size, it can be inferred that the J2 axis needs about twice the torque of J3, in theory, J2 and J3 shafts can not use the same specification of the motor, in order to reduce the J2 axis motor, engineers came up with a good method, that is, by using counterweights to offset part of the torque, so that the entire force system is in a balanced state, In this way, the torque requirements on the motor are significantly reduced.
There are many ways to counterweight, the use of counterweight blocks, the lowest cost, but increase the moment of inertia, the use of spring cylinders, the output is not constant, therefore, only hydraulic cylinders (diaphragm accumulators) such a way is the most ideal.
The following figure shows the use of counterweights in the J3 axis of the robot:
Let's take a look at the KUKA 160kg robot balance cylinder:
Below is the schematic diagram of the KUKA balance cylinder:
The hydraulic balance cylinder used by KUKA robot has a diameter of 40mm and an oil pressure of 20MPa, allowing a load of about 20KN, which means that this balance cylinder will maintain a pulling force of 20kN.
Let's calculate the torque generated on the J2 axis when the robot is loaded at 160kg:
Load pair J2 shaft torque T1 = F (load weight) ×L1 (load arm) = 160kg×9.8N/kg×2m≈3200N.m
Here's how much torque the balancing cylinder can counteract:
The torque of the balance cylinder to the J2 shaft T2 = F (balance bar thrust) × L2 (balance bar force arm) = 20kN× 0.1m = 2000N.m
Here's a look at the torque generated by the 4.3kW motor reducer:
The maximum rotational angular velocity of the J2 axis is w=105°/s (1.83rad/s), and the speed is n=17.5 rpm
T3=9550P/n=9550×4.3/17.5=2346N.m
Therefore, T3>T1-T2, meet the requirements of use, through the above calculations, meet expectations.
Now you should be able to understand why engineers choose balance cylinders, in fact, balance cylinders are not only used in robots, but also in machine tools, truss manipulators, presses and other equipment.
Japanese and German robots are still worth studying and learning, through more looking and thinking, to know what is true, and finally fully master the design and manufacture of robots, if domestic robots can master 10% of the essence, it will not be in this situation.
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