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Grippers 101: A Guide to Gripper Sizing & Selection

by Josh Mayes , Design Engineer
RAD - The Robotic Accessories Leader Posted 12/02/2008

The following guide is provided to help you select the best pneumatic gripper for your specific needs.  First we’ll cover the basics, and then we’ll show you how to calculate your application’s specific requirements and choose the ideal gripper.

Types of Grippers:

Grippers are available in many sizes and shapes for a variety of purposes and payload requirements.

2-Jaw Grippers:  These grippers are the most common, with many variations available.  They provide the versatility to allow handling of almost any shape part.

3-Jaw Grippers:  These grippers are best suited to handle parts with round or cylindrical features.

Collet Grippers:  These specialized grippers are perfect for handling precision parts, typically with cylindrical shaped features, but they can be ordered with any desired standard or custom collet shape.  A key advantage of collet grippers is that they do not require fingers.  Standard collets grip on the O.D., but I.D. expanding collets are also available.

Anatomy of a Gripper:

While there are many types of pneumatic grippers available, most grippers share these common parts:

1. Information you will need to get started sizing your gripper:
(See below for Definition of Terms)

• Part Mass (mpart)
• Finger Mass (mfingers)
• Center of Gravity (CG part) of the part
• Grip on Close (O.D.) or Grip on Open (I.D.)
• Encompassing Grip or Friction Grip
• Part Friction (µ) between gripper fingers and the part
• Stroke needed
• Finger Length required
• Orientation of the gripper during part handling
• Maximum Acceleration/Deceleration (gpart) the part will experience during travel
• Line Air Pressure consistently available
• Design Factor (D.F.) required

2. Calculations to determine your sizing requirements:

For 2-Jaw and 3-Jaw Grippers:

Total Payload (F_tot):  The sum of forces of the part and fingers being handled at maximum acceleration/deceleration.

F_tot = (m_part + m_fingers) x g_part

Grip Force (F_grip):  The minimum force required to securely hold the part at maximum acceleration/deceleration.

If Encompassing Grip, F_grip = F_tot x D.F.
          If Friction Grip, F_grip = [(mpart x 1/µ) + mfingers] x gpart x D.F.

Maximum Moment (M_tot):  The largest moment load imposed on the gripper while the part is being handled at maximum acceleration/deceleration.  This is a sum of two moments - one created by the Total Payload (F_tot) acting on a moment arm Center of Gravity (CG _part), and the second created by the Grip Force (F_grip) acting on the moment arm Finger Length.

M_part = F_tot x CG _part
M_grip = F_grip x Finger Length
M_tot = M_part + M_grip

Example 1: Encompassing Grip on Close

Input	Value
Part Mass (mpart)	5 kg
Finger Mass (mfingers)	2 kg
Center of Gravity (CG part)	75 mm
Grip on Close or Grip on Open	Close
Encompassing or Frictional Grip	Encompassing
Stroke	50 mm
Finger Length	100 mm
Orientation of the Gripper	“Up-Down”
Maximum Acceleration/Deceleration (gpart)	16 m/s2 in X-Y direction†
Available Line Air Pressure	6 bar
Design Factor (D.F.)	1.25

†Typically the X-Y direction refers to a horizontal plane, where Z+ is the upwards direction.  In this scenario, gravity on the part can be neglected.

a. Solve for Total Payload (F_tot):

F_tot = (m_part + m_fingers) x g_part
F_tot = (5 kg + 2 kg) x 16 m/s2
F_tot = 112 N

Note: 1 N = 1 kg?m/s

b. Solve for required Grip Force (F_grip):

F_grip = F_tot x D.F.
F_grip = 112 N x 1.25
F_grip = 140 N

c. Solve for Maximum Moment (M_tot):

M_part = F_tot x CG _part
M_part = 112 N x 75 mm
M_part = 8.4 Nm

M_grip = F_grip x Finger Length
M_grip = 140 N x 100 mm
M_grip = 14 Nm

M_tot = M_part + M_grip
M_tot = 8.4 Nm + 14 Nm
M_tot = 22 Nm

Based on these results, you would need a gripper with a minimum Stroke of 50 mm, at least 140 N Grip Force on Close at 6 bar Line Air Pressure, and the Moment Rating should exceed 22 Nm.

Example 2: Friction Grip on Open

Input	Value
Part Mass (mpart)	12 kg
Finger Mass (mfingers)	4 kg
Center of Gravity (CG part)	200 mm
Grip on Close or Grip on Open	Open
Encompassing or Frictional Grip	Friction, µ = 0.25
Stroke	25 mm
Finger Length	90 mm
Orientation of the Gripper	All
Maximum Acceleration/Deceleration (gpart)	8 m/s2 in all directions†
Available Line Air Pressure	7 bar
Design Factor (D.F.)	2

†It is important to add 1g (9.81 m/s2) for the additional effect of gravity on the part when in its worst case scenario, traveling upward with the gripper jaws oriented in the “Down” position.

a. Solve for Total Payload (Ftot):

F_tot = (m_part + m_fingers) x g_part
F_tot = (12 kg + 4 kg) x (8 m/s2 + 9.81 m/s2)
F_tot = 285 N

b. Solve for required Grip Force (F_grip):

F_grip = [(m_part x 1/µ) + m_fingers] x g_part x D.F.
F_grip = [(12 kg x 1/0.25) + 4 kg] x (8 m/s2 + 9.81 m/s2) x 2
F_grip = 1,850 N

c. Solve for Maximum Moment (M_tot):

M_part = F_tot x CG _part
M_part = 285 N x 200 mm
M_part = 57 Nm

M_grip = F_grip x Finger Length
M_grip = 1,850 N x 90 mm
M_grip = 167 Nm

M_tot = M_part + M_grip
M_tot = 57 Nm + 167 Nm
M_tot = 224 Nm

Based on these results, you would need a gripper with a minimum Stroke of 25 mm, at least 1,850 N Grip Force on Open at 7 bar Line Air Pressure, and the Moment Ratings should exceed 224 Nm.

For Collet Grippers (always a Friction Grip):

Pull-Out Force (F_pull):  The force required to pull a part from the collet.

F_pull = m_part x g_part x Part Friction (µ) x D.F.

Gripping Torque (τ_grip):  The torque required to rotate a cylindrical part in the collet.  Depending on the application, this calculation may be relevant.

τ_grip = F_pull x Part Radius (r)

Example 3: Collet Gripper

Input	Value
Part Mass (mpart)	3 kg
Grip on O.D. or Grip on I.D.	O.D.
Part Friction (µ)	µ = 0.25
Part Diameter	10 mm
Orientation of the Gripper	All
Maximum Acceleration/Deceleration	4 m/s2 in every direction†
Available Line Air Pressure	5.5 bar
Design Factor	1.75

†It is important to add 1g (9.81 m/s2) for the additional effect of gravity on the part when in its worst case scenario, traveling upward with the collet oriented in the “Down” position.

a. Solve for Pull-Out Force (F_pull):

F_pull = m_part x g_part x Part Friction (µ) x D.F.
F_pull = 3 kg x (4 m/s2 + 9.81 m/s2) x 0.25 x 1.75
F_pull = 18.1 N

b. Solve for Gripping Torque (τ_grip): (optional)

τgrip = Fpull x Part Radius (r)
τgrip = 18.1 N x 5 mm
τgrip = .09 Nm

Based on these results, you would need a collet gripper with a minimum Collet I.D. of 10 mm, at least 18.1 N Pull-Out Force at 5.5 bar Line Air Pressure.

3. Choosing the best gripper:

After calculating the requirements of your specific application, you will need to compare your results to specifications provided by the gripper manufacturer to select the appropriate type and size of gripper.

Manufacturer’s Gripper Specifications:

Grip Force:  The available force provided to the part/jaws by the gripper’s pneumatic cylinder, based on supplied air pressure acting upon the piston’s working area.

Grip Force = Line Air Pressure x Piston Area

Moment Ratings (Ma, Mb, Mc):  These are the maximum moment loads the gripper is rated to handle.  Depending on gripper orientation and the direction of acceleration/deceleration, these values can become the criteria for selection of the right gripper for the job.

Total Stroke:  The total distance the jaws move.  If stroke is listed as ‘stroke per jaw’, you will need to multiply this value by 2 for total stroke regardless if it is a 2-jaw or a 3-jaw gripper.  Total stroke is sometimes listed as diametral stroke for 3-jaw grippers.

Grip Force vs. Finger Length:  Usually provided as a chart showing available grip forces at various distances away from the jaw face.  In general, grip force is reduced as finger length increases due to grip-induced moment loading on the gripper jaws/slides.

 
Definition of Terms:

Part Mass (m_part):  The maximum mass the gripper will be required to handle.

Finger Mass (m_fingers):  The total mass of the gripper’s fingers.

Center of Gravity (CG_part):  The maximum distance from the mounting face of the gripper jaws to the part’s mass center.

Grip on Close or Grip on Open:  Will you be gripping the work piece as the jaws close (on the part’s O.D.) or as they open (on the part’s I.D.)?  For many pneumatic grippers, the opening grip force will be slightly different than the closing grip force due to a difference in the cylinder’s effective piston area.  Be sure to confirm the proper rating for your application.

Method of Grip: Encompassing or Friction

An Encompassing Grip wraps around a part.  It does not have to surround the entire part – it can grip around a boss, a groove, or a hole.  With an encompassing grip, the fingers are designed to cradle the part and would need to be pried open in order to drop the part.  An encompassing grip is usually preferred since less grip force is required to grip the part and it provides more stability during movement throughout the work cell.

A Friction Grip relies purely on friction and grip force to securely hold a part.  A friction grip requires more grip force than an encompassing grip.  With a friction grip, it is recommended to maximize surface contact between the part and the gripper fingers for best results.  Note: Collet Grippers always rely on this method of gripping.

Part Friction (µ):  What is the static coefficient of friction between your part material and gripper finger material?  This is critically important when calculating for a friction grip.  The higher value you can achieve through proper finger material selection or coatings, the greater the payload that can be handled securely.  Most people guesstimate their coefficient of friction rather than actually measuring it.  Typical accepted values can be found for a variety of material combinations.  Unless otherwise specified RAD assumes µ = 0.25 for its calculations.  If you are confident that your coefficient of friction is a known value, use that value instead of 0.25.

Stroke:  The stroke of a gripper is simply how far the jaws can move.  Gripping a 5 inch part can often be done with a gripper that has only a 1” or 2” stroke.  You will need to accommodate for the width of the part in the finger design.   If the finger design is for an encompassing grip, the amount that the finger encompasses the part must be subtracted from the useable stroke.

Finger Length:  ‘Finger Length’ is the distance from the mounting face of the jaw to the center of area where the gripper finger contacts the part.  Finger length should be kept as short as possible since the available grip force decreases as finger length increases.  This reduction in grip force is due to grip-induced moment losses.  For this reason, ‘antler’ type fingers should also be avoided where moment forces on the jaws are exaggerated upon gripping.

Orientation:  How will your gripper jaws be oriented throughout the system’s cycle?  In many of today’s robotic applications, the gripper jaws may be oriented in every possible way; however, in simple pick and place applications, the gripper jaw orientation is often fixed, usually in the ‘Down’ direction.  Orientation is important when considering the moment ratings of your gripper.  Certain orientations will handle a given payload better than others.

Maximum Acceleration/Deceleration (gpart):  What is the maximum acceleration or deceleration of the system, whichever is greater, calculated at the part’s Center of Gravity (CG part)?  Typical units are mm/s2, ft/s2, in/s2, m/s2.  To account for this, it is helpful to convert your maximum acceleration/deceleration to the gravitational unit g. If the direction of acceleration is upwards, be sure to add 1g for Earth’s gravity.

1g = 32.2 ft/s² (9.81 m/s²)

Please note: This value should not be confused with maximum system speed.  For gripper sizing purposes, speed is irrelevant.  We only care about how fast the part gets up to speed, or how fast it slows to a stop, causing inertial forces on the part/gripper.

Line Air Pressure:  What is the minimum line air pressure that is consistently available in your facility?  Typically, this value ranges between 60-100 psi (4-7 bar).

Design Factor (DF):  A Design Factor should always be included in your calculations.  The value can vary depending on your application’s design requirements.  Appropriate design factors are based on several considerations including the accuracy of load estimates, the consequences of failure, and the cost of over-engineering the components to achieve a higher factor of safety.  RAD generally uses a Design Factor of 1.6 for sizing purposes.

This document is provided only as a guide to get you started with gripper sizing considerations.  It is not meant to cover every application.  For further assistance, please contact RAD at 937-667-5705 or info@rad-ra.com.

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