Clamping down on every option.

Hydraulic power clamping offers alternative to traditional methods.

Ask an engineer completing a fixture design what the most important and cost effective part of the design process is and he or she will answer planning. All facets of the project should be considered, the designer will continue, and questions answered to help determine the relative cost/benefit of the clamping system chosen before fixture designing begins. This includes alternatives to traditional fixture designing, such as hydraulic power clamping. Sometimes these alternatives work best.

The following information, supplied by SPX Hytec, Owatonna, MN, will help show that a hydraulic power clamping system can be a cost effective fixturing alternative.

Hydraulic force

A basic principle of hydraulics states that pressure applied to a confined fluid is transmitted equally in all directions. This principle allows the transmission of pressure through tubes and hoses to remotely located actuators where that pressure is converted to usable force.

The simplicity of hydraulic power clamping can be summed up in one small equation:

In the typical cylinder pictured, the pressurized fluid at 2000 psi is acting on the 3" sq piston. As the formula says, 2000 psi times 3" sq yields a force of 6000 pounds. This same concept applies to all hydraulic actuators.

The method used to route the pressure to the actuators on the fixture should be determined early in the planning stages. The plumbing is an essential part of the fixture and should never be left as an afterthought. There are two basic plumbing methods; conventional and manifold mount.

Conventional mounting

Conventionally mounted components have threaded ports which accept fittings for tubing and hoses. Many different types of fittings are available, giving the designer several options for customizing his or her design. Since most of these components are commonly available, conventional mounting will typically be the lower cost option.

The threaded ports are usually one of two designs, NPT tapered pipe threads or SAE O-ring boss. NPT tapered pipe threads depend on the interference of the mating thread forms. This thread form has been in use for general plumbing applications for many years. Consequently there is a wide selection of fittings available for even the most unique applications. However, the thread form is the same whether the application is a household water supply or a high pressure hydraulic work holding system. It is important to specify only fittings that are rated for the maximum pressure to be seen in your application. The plastic, copper, and iron pipe fittings are not acceptable alternatives.

Straight thread, O-Ring boss ports per SAE J514 are common in both industrial and mobile hydraulic systems. Because this system of ports and fittings depends on a simple, replaceable oring for sealing instead of the interference of perfectly formed threads, the chance for leakage is greatly reduced.

Systems can be disassembled and reassembled numerous times with no additional make-up required. Fittings will always be in the exact same place and elbows will always point in the right direction. There is never the need to over- or under tighten elbows to properly align them in the system.

Pipe sealants and Teflon tapes that can contaminate the system are not required. The torque needed to properly tighten these fittings is less, too.

All of Hytec's newest products have the SAE ports and a line of fittings and adapters is available from the company's catalog. In addition, the company has made many of its other products available with SAE ports.

Manifold mounting

Manifold mounted components eliminate the need for external fittings, tubing, and hoses because the fluid passages are machined directly into the fixture. Securing the work holding component to the fixture automatically makes the hydraulic connection.

* Provides no-tool hydraulic connections.

Manifold mounting:

* Saves valuable fixture space.

* Eliminates tubes, hoses, or fittings that disrupt coolant flow and collect chips.

* Simplifies post-machining fixture cleaning.

* Reduces assembly and maintenance time.

* Improves performance.

* Means fewer hydraulic connections resulting in fewer potential lead points.

* Results in a cleaner, more professional looking fixture.

Plumbing sizing

When designing and assembling the hydraulic system, designers should keep in mind that their choices of size and length plumbing lines can significantly change the performance of the fixture. The back-pressure created by fittings, tubing, and hoses can slow the operation of the system, especially single-acting systems. Larger diameter plumbing runs, with a minimum number of bends and fittings, will reduce this back pressure.

When sizing hydraulic lines, designers should make sure to look at the inside diameter. Hose labeled as 1/4" is not the same as '/4 tubing. Hose is specified by its inside diameter; hydraulic tubing is usually specified by the outside diameter. Tubing that is 1/4", CD 0.035", has an inside diameter of 0.180", which is a flow carrying capacity of only 50% of that of the hose.

Single acting clamps can develop only a limited amount of pressure to force hydraulic fluid out of the clamp and allow it to retract. When the return fluid from multiple clamps must share the same hydraulic line, back pressure can easily become excessive and slow the clamp's retraction.

When connecting multiple clamps, designers can use either a "daisy chain" or "home run" configuration. In a daisy chain, designers use a tee at each clamp and run tubing from the first clamp to the second, and then to the third and the fourth, and so on. When using a home run configuration, designers begin at a manifold and run hydraulic lines all the way from the manifold to each clamp.

The daisy chain method uses less tubing, so it might appear that this would minimize back pressure. However in the daisy chain, the fluid from all of the clamps must pass through a single hydraulic line. In the home run, while there may be longer runs, each line has to accommodate flow from only one clamp.

The viscosity of the hydraulic fluid used will also affect back pressure. Viscosity is affected by temperature. Designers should contact the factory to discuss applications running below room temperature. The company recommends using only Hytec fluids when using the company's products. Other fluids may have different viscosities or other characteristics that can adversely affect system operation.

Single- vs double-acting

Another decision is whether to use single- or double-acting components. Single-acting components are typically actuated using hydraulic pressure. When released, the pressure is removed and the actuator is returned by a spring which forces the hydraulic fluid back into the pump reservoir. This type of system is usually the most cost effective because each actuator needs only one pressure source connection for operation. Remember, double the plumbing for double-acting systems. This does, however, use more valuable fixture space and adds to the cost.

There are good reasons to use double-acting systems. The larger and/or more complex the circuit design, the greater the potential for return restrictions which will slow the return of the single-acting actuators. Double-acting actuators are ideal for applications which require both pushing and pulling or returning clamps with heavy, custom designed attachments. They work well for powering linkages that require fast actuation in both directions. Double-acting clamps are often used in automated systems where coordinating the action of the clamp with that of the rest of the system requires fast, positive, predictable cycle times. By installing pressure switches in both the pressure and return lines, the status of the clamp can constantly be monitored.

Hydraulic power clamping also provides varying degrees of automation. During the planning stage, the method of actuating the fixture must be considered. The simplest systems use manually operated valves where the operator turns a handle to clamp and unclamp the fixture. In totally automated systems, the machine tool itself can be programmed to control the clamping and unclamping functions through the use of electric solenoid valves.

Positioning vs clamping

Hydraulic actuators are typically used on a fixture to perform one of two functions: positioning or clamping. Positioning actuators' primary purpose is to push the workpiece against the solid positioning stops built into the fixture. Clamping actuators hold the workpiece in position during machining.

With a properly designed fixture, all the operator needs to do is to place the workpiece into the fixture. The positioning actuators (typically cylinders) will move and correctly orient the workpiece against the stops, and hold it there while the clamps are sequenced, thus securing the part to resist machining forces. While clamps are always needed to hold the part, positioning actuators are sometimes optional depending on the workpiece, fixture design, and the level of operator involvement.