Most of this chapter has been concerned with grippers rather than tools as end effectors. As indicated in Sec. 5.4, tools are used for spot welding, arc welding, tooling used with these operations when we discuss the corresponding applications in Chaps. 14 and 15. In this section, let us summarize our discussion of grippers by enumerating some of the considerations in their selection and design.
Certainly one of the considerations deals with determining the grasping be considered in assessing gripping requirements. The following list is based on Engelberger’s discussion of these factors:
2. The size variation of the part must be accounted for, and how this might problem in placing a rough casting or forging into a chuck for machining operations.
3. The gripper design must accommodate the change in size that occurs between and forging operations.
4. Consideration must be given to the potential problem of scratching and distorting the part during gripping, if the part is fragile or has delicate surfaces.
5. If there is a choice between two different dimensions on a part, the larger dimension should be selected for grasping. Holding the part by its larger surface will provide better control and stability of the part in positioning.
This provides better part control and physical stability. Use of replaceable models.
A related issue is the problem of determining the magnitude of the grasping force that can be applied to the object by the gripper. The important factors that determine the required grasping force are:
The weight of the object. Consideration of whether the part can be grasped consistently about its center of mass. If not, an analysis of the possible moments from off-center grasping should be considered.
deceleration forces), and the orientational relationship between the direction of
Whether physical constriction or friction is used to hold the part.
We have discussed the methods for dealing with these factors and analyzing the gripping forces in Secs. 5.2 and 5.3. Table 5.1 provides a checklist of the many different issues and factors that must be considered in the selection and design of robot gripper.
Table 5.1
Factor Consideration
Part to be handled Weight and size
Shape
Changes in shape during processing Tolerances on the part size
Surface condition, protection of delicate surfaces
Actuation method Mechanical grasping
Vacuum cup Magnet
Other methods (adhesives, scoops, etc.) Power and signa transmission Pneumatic
Electrical Hydraulic Mechanical
Gripper force Weight of the object
(mechanical gripper) Method of holding (physical constriction or friction) Coefficent of firction between fingers and object Speed and acceleration during motion cycle
Positioning problems Length of finger's
Inherent accuracy and repeatability of robot Tolerances on the part size
Service conditions Number of actuations during lifetime of gripper Replaceabilty of wear components (fingers) Maintenance and serviceability
Contd.
Operating environment Heat and temperature
Humidity, moisture, dirt, chemicals
Temperature protection Heat shields
Long fingers
Forced cooling (compressed air, water cooling, etc.) Use of heat-resistant materials
Fabrication materials Strength, rigidity, durability Fatigue strength
Cost and ease of fabrication Friction properties for finger surfaces Compatibility with operating environment Other considerations Use of interchangeable fingers
Design standards
Mounting connections and interfacing with robot Risk of product design changes and their effect on the gripper design
Lead time for design and fabrication Spare parts. maintenance, and service Tryout of the gripper in production
P
5.1
surface is estimated to be 0.3. The orientation of the gripper will be such that the weight of the part will be applied in a direction parallel to the contacting g factor to be used in force calculations should be 3.0. Compute the required gripper force for the 5.2
5.3 g factor of 3.0, the following
information is given to make the force computation. The robot motion cycle has been analyzed and it has been determined that the largest acceleration
of 1.5, compute the required gripper force.
5.4 A part weighing 15 lb is to be grasped by a mechanical gripper using friction
Why? Compute the required gripper force assuming that a g factor of 2.0 is applicable.
5.5 -
mine the required actuating force if the gripper force is to be 25 lb.
Fig. P5.5
5.6 -
culate the required actuating force if the gripper force is to be 20 lb.
Fig. P5.6 5.7
and the diameter of the suction cup is 6.0 in. Determine the negative pressure
2) to lift each plate.
Use a safety factor of 1.5 in your calculations.
5.8 A vacuum pump to be used in a robot vacuum gripper application is capable of drawing a negative pressure of 4.0 lb/in.2 compared to atmospheric pressure.
The gripper is to be used for lifting stainless steel plates, each plate having dimensions of 15 by 35 in. and weighing 52 lb. Determine the diameter of the suctions cups to be used for the robot gripper if it has been decided that two suction cups will be used for the gripper for greater stability. A factor of safety of 1.5 should be used in the design computations.
5.9
stroke. The inside diameter of the piston is 2.0 in. and the ram diameter is force? Use a safety factor of 1.3 in your computations.
5.10
the anticipated g factor is 2.0. The gripper is to be operated by a piston whose
Fig. P5.10
(a) Determine the required gripping force to retain the part.
(b) Determine the actuation force that must be applied to achieve this gripping force for this mechanical design.
(c) Determine the air pressure needed to operate the piston so as to apply the required actuation force.
(d) If a safety factor of 1.5 were to be used for this design, at what point in the computations would it be appropriate to apply it?
, 3rd ed.,
4. M. P. Groover and E. W. Zimmers, Jr., :
,
6. J. E. Shigley and L. D. Mitchell, , McGraw-Hill.