How to Choose the Right Grips for Tensile Testing
- christopherthomasb
- 35 minutes ago
- 6 min read
Wedge vs. Pneumatic vs. Screw Action Grips
Selecting the correct grips for tensile testing is one of the most important — and most overlooked — decisions in any materials testing setup. While universal testing machines often receive the most attention, the grips are the component that directly interface with the specimen, making them a primary determinant of test accuracy, repeatability, and compliance with ASTM and ISO standards.
Engineers frequently ask: “What grips should I use for tensile testing?” The answer depends on more than just load capacity. Material type, specimen geometry, surface finish, test standard, and throughput requirements all play a role. Choosing the wrong grips can result in specimen slippage, jaw breaks, invalid results, or even damage to the testing machine itself.
In this guide, we’ll walk through the three most common tensile grip types — wedge grips, pneumatic grips, and screw action vise grips — and explain how to choose the right option for your application.
Why Choosing the Right Tensile Grips Matters
Tensile testing problems are often blamed on the test method or the specimen, but in many cases the root cause is improper gripping. Grips must apply force evenly, maintain alignment, and securely hold the specimen without introducing stress concentrations.
Incorrect grip selection can lead to:
Specimen slippage during loading
Premature failure at the jaws instead of within the gauge length
Damage to soft or delicate materials
Inconsistent results between operators
Non-conformance with ASTM testing requirements
Using the proper tensile grips improves data integrity, operator safety, and test efficiency, while reducing scrap and re-testing costs. This is why experienced labs treat grips as a critical system component — not an accessory.
What Are Tensile Testing Grips?
Tensile grips are mechanical or pneumatic devices that secure a test specimen to a universal testing machine so that axial tensile force can be applied in a controlled and repeatable manner.
A complete gripping system typically includes:
Upper and lower grip bodies
Jaw faces or inserts that contact the specimen
Alignment features to ensure axial loading
Interfaces compatible with the test frame or load cell
Grips must transmit load without slipping or damaging the specimen while maintaining alignment throughout the test. Jaw inserts — often overlooked — play a major role in how force is distributed and whether a test passes or fails.
Key Factors That Determine Which Tensile Grips You Need
Before choosing a grip type, it’s important to evaluate the specifics of your application. The following factors should guide every tensile grip selection:
Material Type
Different materials respond very differently to clamping force:
Metals and rigid composites tolerate high contact pressure
Plastics and elastomers deform easily and require controlled gripping force
Films, foils, and textiles are especially sensitive to surface damage
Specimen Geometry
Common specimen shapes include:
Flat dog-bone specimens
Round rods or wires
Thin films or sheets
Grip design and jaw insert geometry must match the specimen shape to ensure uniform load transfer.
Force Range
Low-force testing requires precise control to prevent crushing or slippage, while high-force testing demands grips that can withstand significant loads without deflection or wear.
Test Standard Requirements
Many ASTM and ISO tensile standards specify grip alignment, gripping length, or specimen failure location. Selecting grips compatible with these requirements helps ensure compliance and audit readiness.
Throughput and Repeatability
High-volume labs benefit from grips that offer fast setup and consistent clamping force between operators.
Wedge Grips: The Most Common Tensile Grip Solution
What Are Wedge Grips?
Wedge grips are mechanical, self-tightening grips that use angled jaws to increase clamping force as tensile load increases. As the test progresses, the applied load causes the jaws to seat more tightly against the specimen.
This simple but effective design makes wedge grips one of the most widely used solutions in tensile testing.
Advantages of Wedge Grips
Wedge grips are favored for many applications because they offer:
High gripping force suitable for metals and rigid materials
Self-tightening action that reduces manual adjustment
Excellent durability for repeated high-load testing
Compatibility with a wide range of jaw inserts
They are commonly used in testing metals, composites, and rigid plastics, and are well suited for standards that involve higher forces.
Limitations of Wedge Grips
Despite their versatility, wedge grips are not ideal for every application:
The increasing clamping force can damage soft or thin materials
Less precise control over gripping pressure
Not ideal for delicate films, elastomers, or low-force testing
When to Choose Wedge Grips
Wedge grips are typically the best choice when testing:
Metals and metal alloys
Rigid plastics
Fiber-reinforced composites
Medium- to high-force specimens
Universal Grip’s UG and UGS wedge grip systems are designed for durability, precise alignment, and interchangeable jaw inserts, making them adaptable to a wide range of tensile testing applications.
Pneumatic Grips: Precision and Repeatability for Sensitive Materials
What Are Pneumatic Grips?
Pneumatic grips use compressed air to apply a controlled and constant clamping force to the specimen. Unlike wedge grips, pneumatic grips do not increase clamping pressure as load increases, making them ideal for materials that are sensitive to surface damage or deformation.
Advantages of Pneumatic Grips
Pneumatic grips offer several key benefits:
Consistent gripping force independent of applied load
Reduced risk of specimen damage
Excellent repeatability between operators
Faster specimen loading and unloading
Ideal for automated or high-throughput testing environments
Because the gripping pressure can be precisely regulated, pneumatic grips are commonly used for plastics, films, elastomers, and other compliant materials.
Limitations of Pneumatic Grips
Pneumatic grips do have some considerations:
Require a clean, stable compressed air supply
Higher upfront cost compared to mechanical grips
Typically lower maximum force capacity than wedge grips
When to Choose Pneumatic Grips
Pneumatic grips are an excellent choice for:
Plastics and polymer testing
Thin films and sheets
Rubber and elastomers
Applications requiring high repeatability and throughput
Universal Grip offers pneumatic grip solutions with a variety of jaw options and pressure ranges, allowing labs to tailor gripping force to the specific material being tested.
Screw Action Grips: Control and Versatility for Low-Force Testing
What Are Screw Action Grips?
Screw action grips rely on manually tightened screws or bolts to clamp the specimen in place. The operator controls the gripping force directly, making these grips simple and versatile for low-force applications.
Advantages of Screw Action Grips
Screw grips are valued for:
Simple, intuitive operation
Low cost
Fine control over clamping pressure
Compatibility with a wide range of specimen sizes
They are often used in research labs, educational settings, or specialized low-force testing applications.
Limitations of Screw Action Grips
Because screw grips rely on manual tightening:
Repeatability can vary between operators
Setup time is slower compared to pneumatic grips
Not suitable for high-force testing
Increased risk of uneven clamping if not carefully adjusted
When to Choose Screw Action Grips
Screw action grips are best suited for:
Thin films and foils
Textiles and fibers
Low-force tensile testing
Prototyping and R&D environments
Universal Grip supplies screw action grips designed for precise alignment and interchangeable jaw inserts, making them a reliable option for controlled, low-force testing.
Jaw Inserts: The Most Overlooked Component in Tensile Testing
Jaw inserts are the surfaces that actually contact the specimen, and they have a major impact on gripping performance. Even the best grip body will fail if paired with the wrong jaw inserts.
Common jaw insert types include:
Serrated jaws for hard materials that require aggressive gripping
Smooth jaws for softer materials
Rubber-coated jaws for delicate specimens
Carbide-coated jaws for increased wear resistance
Custom-machined inserts for unique specimen geometries
Selecting the correct jaw insert helps prevent slippage, reduces specimen damage, and ensures failures occur within the gauge length as required by most tensile standards.
Universal Grip offers a broad range of standard and custom jaw inserts to match virtually any tensile testing application.
Common Tensile Grip Problems — and How to Fix Them
Even experienced labs encounter gripping issues. Understanding common problems can help prevent costly downtime and invalid tests.
Specimen Slippage
Often caused by insufficient gripping force or inappropriate jaw inserts. Switching jaw materials or grip type usually resolves the issue.
Breaks Outside the Gauge Length
Typically the result of stress concentrations at the grips. This may indicate excessive clamping pressure or poor alignment.
Jaw Damage or Excessive Wear
Hard materials can quickly wear standard jaw faces. Upgrading to hardened or carbide-coated inserts extends service life.
Misalignment
Misalignment introduces bending stresses that compromise test results. Proper grip alignment and self-centering features are essential.
Wedge vs. Pneumatic vs. Screw Grips: Quick Comparison
Grip Type | Best For | Force Range | Repeatability | Setup Speed |
Wedge Grips | Metals, rigid materials | High | High | Medium |
Pneumatic Grips | Plastics, films, elastomers | Medium | Very High | Fast |
Screw Action Grips | Thin or delicate specimens | Low | Medium | Slow |
Final Thoughts: Talk to a Tensile Grip Specialist
No two tensile testing applications are exactly alike. Material properties, specimen geometry, force requirements, and test standards all influence grip selection. While this guide provides a solid framework, working with a grip and fixture specialist can prevent costly mistakes and ensure long-term testing success.
Universal Grip specializes in wedge grips, pneumatic grips, screw action grips, and custom jaw inserts, helping labs configure tensile testing systems that deliver accurate, repeatable, and standards-compliant results.
If you’re unsure which grips are right for your application, consulting a specialist before purchasing can save time, money, and frustration — and ensure your tensile testing data stands up to scrutiny.
Visit us at www.universalgripco.com
or call 978-882-1480