Hydraulic workholding from scratch

How to select and set up components

STAMPING Journal July/August 2001
December 13, 2001
By: Thomas Eggert

Hydraulic clamps are used in place of manual or toggle clamps to hold the workpiece or tool in place during stamping. This article examines the four steps that comprise the selection and installation of a hydraulic workholding system: cylinder selection, cylinder force and stroke, power source selection, and system connection.

Hydraulic clamps can be used in place of manual or toggle clamps to hold a workpiece or tool in place during stamping. The foundation for effective hydraulic workholding is built on component selection and system connection.

Whether the system is simple or complex, the job at hand basic or sophisticated, four simple steps — three dealing with equipment options and one with system setup — comprise the selection and installation of a hydraulic workholding system.

Step 1: Cylinder Selection

Proper cylinder selection is the most critical factor in any workholding system. The shape and size of the workpiece and the machining process involved will determine the type and size of cylinders most suitable for the job. A range of production tooling cylinders are available for various tasks:

1. Positioning and push cylinders guide the workpiece and push it securely into the desired position.

2. Down-holding cylinders, including swing cylinders and edge clamps, clamp the positioned workpiece firmly to the fixture or worktable.

3. Pull cylinders, some available with hydraulic or spring return, clamp the workpiece or fixture in applications requiring pull forces.

4. Work support cylinders maintain the workpiece accurately on the prescribed plane during machining.

Swing cylinders and work supports often are used in combination for a variety of workholding applications. Swing cylinders typically are used as clamping components in applications requiring unrestricted loading and unloading. Providing maximum clamping force in a small package, they enable unobstructed part fixturing and placement.

A swing cylinder's rod and attached clamp arm rotate 90 degrees in clockwise or counterclockwise directions, then travel down an additional distance to clamp against the fixtured part. When the clamping pressure is released, the clamp arm rotates 90 degrees in the opposite direction to allow for part removal and new-part placement.

Different mounting options and operation styles enable users to select components to suit almost any application. Varieties include upper- and lower-flange swing cylinders, as well as threaded-body and cartridge-model swing cylinders. Different sizes and styles of clamp arms also are available.

When the machining operation requires positive hydraulic return action, double-acting cylinders can be specified because they provide a fast release time to help increase output. If spring-return action is sufficient, single-acting cylinders or a combination of single-acting and double-acting units can be used.

Step 2: Cylinder Force and Stroke

The size and shape of the workpiece and the designated machining operation generally determine the cylinder force and stroke, as well as the size and style of work supports. Still another influence is the work space or clearance around the job at hand.

During fixture design, several features of swing cylinders and work supports should be taken into consideration. Perhaps the most important is the amount of support force needed, which, in turn, will influence the size of the work support. In principle, the work support must overcome both the clamping forces and the machining forces, including those that may be generated by vibrations.

Usually the clamping force applied to the work support should not exceed 50 percent of its capacity at a given operating pressure. For many applications, this is sufficient to absorb additional forces, such as machining forces. The 2-to-1 safety factor may need to be increased to 4-to-1 if extreme vibration or an interrupted cut is part of the application.

The recommended ratio between clamping force and support forces can be achieved by selecting the right sizes of clamping components or by operating the swing cylinder and the work support with different operating pressures.

Step 3: Power Source Selection

Whether parts are needed once a day or 24 hours a day, the power source for an automatic workholding system should be carefully, accurately matched to the job. Sizes and capacities should match job requirements, and the type of pump should correspond to the application and work environment. Available styles include:

• Air-hydraulic pumps that generate hydraulic pressure using available air pressure. These pumps provide the desired power and speed in simple clamping circuits and are suitable for medium- and lower-cycle applications. Many models have a modular design, so valves can be added to meet simple or complex application requirements.

• Electric pumps designed for applications involving high cycle life and requiring higher work flows. Single- and two-stage electric workholding pumps can be customized to meet continuous-duty cycles and other application requirements.

A variety of accessories, including return line filter kits, heat exchanger kits, float/temperature switch kits, pressure switch kits, and manifold kits, make further customization possible.

Step 4: System Connection

After evaluating the application requirements and identifying the basic components needed to meet them, the user must select connecting pieces to put the system together. This involves connecting the pump to the various control valves and cylinders through a circuit of hoses and/or piping, fittings, gauges, and accessories.

As with the major components, connecting pieces should be selected to suit the job. A mismatch can impair performance, shorten operating life, or compromise safety.

Basic follow-through requires thorough evaluation of all of the parts in the system. Manufacturers' catalogs often provide helpful assistance by listing or illustrating compatible components and accessories for different applications.

As an example, two swing cylinders and work support cylinders working in sequence and powered by an electric-drive hydraulic pump unit would require the following components: a gauge and gauge adapter to monitor system pressures and maintain process accuracy, high-pressure hydraulic hoses with a heavy-duty protective plastic coating, and leak-free fittings to connect all cylinders.

Knowing the components and the roles they play, as well as the way they should be sized and selected, will help contribute to a dependable, safe, productive hydraulic clamping system operation.

Thomas Eggert

Contributing Writer

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STAMPING Journal is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping.

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