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Robotics Tech Papers

Design Guidelines for Pneumatic Grippers

by Design Guidelines for Pneumatic Grippers, Design Guidelines for Pneumatic Grippers
American Grippers Inc.

When designing a robotic or automated assembly machine, applying a gripper correctly unit can make the difference in the reliability and productivity of the assembly machine system or robotic cell. Gripper calculations will vary depending on many factors such as, part acceleration, part weight, physical size, tooling length, tooling jaw style (encompassing grip or friction grip), tooling jaw material and the number of jaws that come in contact with the part. Below are some fundamental factors to keep in mind when designing automated equipment with a gripper. Remember the gripper is the last part of the machine to handle the part being assembled, so careful consideration should be made.

Main considerations:

1)      Tooling jaw friction calculation using Coulomb friction (Ff = mFn) as a simple model where: Ff is force of friction exerted by each tooling jaw surface, parallel and opposite to the net applied force, m is the coefficient of friction, and Fn is the normal force exerted by each tooling jaw on each other. The maximum value of static friction that can be experienced through the surface of contact equals, where is the limiting coefficient of static friction and is the normal force.

2)      Gripping force factor calculation guidelines are made to ensure that the actuator mechanism will apply the proper force and prevent slipping or dropping or parts being handled, design for worst case conditions. More specific calculations are: mnf Fg = wgS where m = coefficient of friction, nf = number of tooling jaws, Fg = gripper force, w = part weight, g = gravity and S = safety factor (3 to 4 in a typical application, higher speeds will require a greater safety factor).

3)      Keep tooling finger length to a minimum to keep moment loads and forces to a minimum. The longer the tooling the greater the rotation forces are created at the gripper bearing reducing its force applied to the part or efficiency to translate piston force to jaw force. Longer jaw tooling will reduce useful life by causing excessive wear on the actuators moving components. Follow maximum finger length charts included in manufacture data sheet to maximize gripper life.

4)      Design tooling to encompass part, this will ensure a secure grasp of the part being handled. Consideration should be taken not to cause a wedging condition, but to constrain the part in both x and y plane. When manipulating several similar parts a dual V configuration can be used to grasp several different parts, note to avoid centerline shift use four point contact. Take in to account surface friction of part being manipulated as well as the material chosen for tooling fingers. Note that in any gripper application the tooling material chosen as well as the gripper jaw bearing will flex, so the tooling design should account for this.

5)      Torque seen by parallel gripper is calculated by the sum of all torques, by adding part torque and jaw torque to have a clear picture. Part torque would be jaw length x part weight x acceleration, jaw torque is closing force of gripper x jaw length. Do not forget to add the weight of your tooling jaws and find the (center of gravity “CG”). To minimize this torque keep work piece as close to gripper as possible and lighten tooling jaw to keep inertia at a minimum.

6)      Compliance in gripper or in part fixture will also increase gripper life by keeping stresses off gripper bearings and internal parts. Inaccuracies can come from part tolerance, precision of gripper, alignment of part from pick to final place position. Keep in mind as the machine ages there will be a degradation of repeatability throughout the machine as components wear.

7)      Apparently obvious but sometimes forgotten when designing grippers in assembly systems are allowing for tooling clearance on approach of part and relevant clearance from full close to open position at placement in a fixture to avoid interference. Both cases this can cause failure at debug or run off of a machine and damage components. Not as apparent but still important is not relying on location of parts added earlier in the assembly process, sensors or vision system can detect misalignment and prevent crash situations.

8)      Correct part spacing between tooling jaw and part will increase throughput, this is important in any pick and place application to increase speed and reduce misaligned or jammed parts.

9)      One of the most important design considerations when working with grippers is to manipulate the part without releasing it. This will increase reliability and decease any opportunity for error due to dropping or mishandling the part.

Pneumatic gripper selection
The two main styles of grippers are parallel and angular referring to jaw movement. Parallel grippers are the most common due to the ease of tooling, adapt to various part sizes without changing tooling finger and can be used non-synchronous to allow jaws to comply and shift to work piece centerline. Two jaw gripper are use in most of the applications since they are cost effective, can handle multiple shapes and sizes. Three jaw grippers are mostly used on round parts that are handled axially and offer more gripping force with shorter strokes. Angular gripper jaws swing an arch that can be adjusted to reduce the opening swing; usually they are dedicated to picking up one size part and useful where vertical space is limited. They are also offered in two or three jaw configurations with synchronized jaw movement and come in fail safe (toggled locked jaw) or standard non locking types. Angular grippers with 180 degree jaw travel need to take into account the inertial at the end of stroke, keeping tooling weight and acceleration down. This can be accomplished by the use of light weight materials like aluminum, shorten tooling jaw as much as possible and the use of flow controls to reduce speed of tooling swing. The other types of standard grippers are made with a specific task in mind, such as the O-ring gripper to aid in the assembly of installing o-rings on parts or the single jaw gripper when a zero positions is needed.

In conclusion
These are basic tools and suggestions and every application is unique. The loads that grippers handle vary greatly do to part weight, size, speed of movement, material type, shape of tooling jaws and shop air pressure. This design guild is made to help you narrow down your selection and increase the reliability, productivity of the assembly machine system or robotic cell. To learn more concerning our assembly automation components, please call or e-mail us and we will help with your application needs.

 

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