Most manufacturers of gripping devices offer their products in both parallel and angular jaw models. This choice is typically made based upon the geometry of the part and the number of parts that will be gripped.
While angular jaw travel grippers tend to be the least costly, they also tend to be the least versatile when tooling is applied. If the device will always be gripping the same part at the same position, and the size of that part doesn't vary by any great degree, then properly designed tooling on an angular jaw motion gripper can be the best choice.
If the size of parts can vary or if the requirement is to be able to accommodate more than one part on demand, then perhaps a better choice would be a parallel jaw motion device. Many designers believe that a parallel jaw motion is the only safe way to interface with round parts, but a properly tooled angular motion can be just as effective.
When deciding on angular or parallel jaw motion, it is also important to determine part encapsulation and total jaw travel. These two aspects are very closely related during sizing and selection. Encapsulation refers to whether the gripped part will be mechanically interlocked by the gripper jaw tooling or a simple surface-to-surface friction grip between the jaw tooling face and the user's part.
When sizing a gripper, required safety factors usually take encapsulation into account. Generally, a higher safety factor should be used when relying strictly on a frictional grip. Also, a coefficient of friction between the jaw tooling and the part being gripped is required to calculate the resulting frictional grip force.
Pneumatic grippers are available in many sizes and are offered in both parallel and angular jaw models.
The other attribute, total jaw travel, is the distance between the two jaws when fully open, minus the distance when both jaws are closed. Consideration for total jaw travel is dependent upon the required distance of encapsulation, the clearance required when approaching the part to be gripped, and the variation of part sizes for the application. Most gripper manufacturers offer one or two series of products where total jaw travel is a premium feature for that series.
Payload, pressure, and tooling length
Eventually, the designer must consider one of the major criteria for selecting and sizing an automation gripper, that being the payload. This includes not only the weight of the item that is being transferred, but also the total weight of all attached tooling. Designing gripper applications using the lightest material possible for tooling is almost always desirable. This not only decreases external moment forces that are reflected back into the gripper and its mechanics, it also increases the amount of ultimate usable payload that can be carried by the rest of the automation system.
For example, using aluminum or high-strength plastic tooling as opposed to steel can be a significant reduction in total weight being carried. Also with pneumatic grippers, the grip force is a function of the pressure applied. Basically, the higher the input pressure, within rated capacity, the greater the grip force developed.
Another significant consideration is the length of tooling used. For angular grippers, the amount of grip force developed is usually inversely proportional to the length of the jaw tooling. For many parallel style grippers, the length of tooling will result in a derating value used to calculate the total grip force developed.
Fortunately for today's machine designer, many gripper manufacturers offer some form of sizing software that greatly simplifies the gripper sizing and selection process for most any automation application. By filling in the blanks and selecting criteria that applies to their application, the user can obtain a list of gripper models and sizes that will successfully accomplish the task required. Some of these sizing tools are available online and offer the user the convenience of inputting their application parameters in either imperial or metric units, or a mix of both.
Some sizing tool packages, such as the one offered by PHD Inc., allow the user to select two or three different models side by side and compare the resulting specifications of one to another. This comparison mode allows the designer to tailor the gripper selection to the optimum device for the application.
Over the years, the automation industry has come a long way in assisting machine designers, and the ability to successfully apply cost-effective grippers has never been easier.
— Jon Dent is distributor training and development manager for PHD.