power actuated die clamps - smith & · pdf filepower sources used to actuate die clamps...

Power Actuated Die Clamps C139- doc © 1990-2006 Rev July 19, 2006 Smith & Associates, 530 Hollywood Drive, Monroe, Michigan 48162-2943

Power Actuated Die Clamps Fully automatic die changing systems require some type of power actuated die clamping system. Semi-automatic and manual die changes also can be performed more rapidly in many cases by the use of power die clamps. Power Sources Used to Actuate Die Clamps Power actuated die clamps can be actuated by several energy sources, either singly or in combination. These include:

1. Electrical energy 2. Compressed air 3. Hydraulic pressure

Electrically Powered Die Clamps Some clamps actuated by electrical motors are in use, although they are uncommon. The required gear reduction and electrical control system are often complex and expensive. Compressed Air Powered Clamps Clamps powered by compressed air have been in use for many years. An early Air Powered Clamp developed by the Danly Machine Company featured an over center toggle eccentric mechanism. This device provides a large clamping force in the locked position. A limit switch to indicate locked/unlocked status is an important safety feature. It is essential to machine and locate the upper die clamping slots accurately to avoid clamp interference. The die ledge or plate thickness must be precisely machined. A slight variation in this height will drastically change the clamping force. Hydraulic Clamp Advantages Hydraulic clamps are used for both new press installations and retrofitted to existing operations. It is important to understand that hydraulic clamps are not a universal requirement for quick die change. Automatic clamps should be considered whenever die change times are less than 20 minutes. However, power clamps should be considered whenever manual bolting is not achieving sufficient press uptime. In addition, variations in manual bolting torque may not attain safe and consistent die retention forces. Torque variation can be a cause of process variability in some operations. Of course, anytime manual bolting cannot be accomplished safely and consistently; power clamping may be a good alternative. Hydraulic Pressure Sources Small to medium sized systems most often employ air over oil intensifier pumps to provide hydraulic pressure. These systems are simple and dependable provided normal maintenance is performed. Large systems such as those on transfer presses having many

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clamps that are applied simultaneously require large volumes of pressurized hydraulic fluid. Here, an electrical motor driven pump may be used to supply the required volume. A hydraulic accumulator may be used to speed up the clamping time. A low-pressure hydraulic switch is a normal safety feature of both air-over-oil and motor driven hydraulic pressure sources. By monitoring for a loss of hydraulic pressure, a number of failure modes can be detected and the press stopped. Low-pressure safety switches vary in their setpoints—80% of the system operating pressure is typical. General Safety Considerations All automatic systems should be of fail-safe design in that the die cannot become detached or shift position on the ram or bolster during press operation. This could occur due to a failure of the clamping power source or by command to release the clamps while the press is in motion. To avoid this occurring, good designs incorporate some or all of these safety features:

1. Dual hydraulic power sources across diagonal corners of the machine much like the dual brake system on automobiles.

2. Pressure switches to detect a loss of holding force. 3. Automatic machine shutdown in case of the loss of pressure. 4. Over-center toggle locking mechanisms that will hold in case of a loss of

pressure. 5. Hydraulic clamps with built-in check valves to prevent release of fluid in case of

a pressure failure--these require a second hydraulic line to release the pilot-operated check valve.

6. Employment of the wedge and ramp locking principle used to retain tapered

shank drills and reamers in standard holders. This type of clamp requires the application of pressure to a second hydraulic line to unlock the wedges and release the clamp.

7. Limit switches to detect proper clamp position. In addition, proximity switches

can monitor that the die and press slide is in the proper position for clamping. Safety is the paramount concern in any die clamping system. Should the die shift or fall out of the press, severe die and/or press damage is very likely to occur. Over the years many-unexpected and needless pressroom injuries have occurred. Many of these accidents are attributable to clamping system failures that could have been prevented by following proper system design and use.

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Hollow Piston Cylinder Clamp Figure 1 illustrates a type of clamp that is essentially a hydraulically powered nut. The hydraulic unit screws onto a standard T-slot bolt. The piston in the hydraulic unit applies downward pressure against the die-clamping surface (not shown) in the same way as a conventional nut when tightened. The face of the piston is equipped with a captive spherical swiveling washer to compensate for irregular and tapered clamping surfaces. A hose attached to the cylinder port supplies hydraulic pressure. A variation of this type clamp uses a compact type of spring contains a stack of dish shaped Belleville spring washers spring washers to supply the holding force. The clamp releases by the application of hydraulic pressure.

Figure 1. A cylinder designed for use with a standard T-bolt is popular for hydraulic clamping retrofit applications. The cylinder essentially functions as a hydraulically powered nut. Design for drawing courtesy of Hilma Division of Carr-Lane Roemheld. Smith & Associates This design is popular for quick changeover retrofits. A major advantage is that the clamp can be adapted to a variety of clamping heights in the same manner as a screw and nut are adjusted. When adjusting the position of the hydraulic nut on the screw, it is very important to make sure that the piston travel is sufficient. To avoid problems, use a constant clamping height for all dies. The hydraulic nut may be pinned in place to avoid misadjustment. Instruct all personnel responsible for safe press operation to be sure that there is sufficient piston travel including a safety factor.

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Engineering Retrofit Applications Whenever a hydraulic nut clamp is specified to replace a standard bolting system, a simple engineering analysis should be conducted to insure safe die retention. It is extremely important to be sure that the same clamping forces known to safely retain dies with conventional bolt and nut die fastening are developed and maintained by the hydraulic clamps. For example, if a die is retained by standard grade eight T-slot bolts, a hydraulic pressure of approximately 6,000-psi (41,364 kPa) may be required to provide equivalent die retention. If correct engineering calculations are neglected, someone may assume that a 2,400-psi (16,546 kPa) hydraulic pump is sufficient. The lower pump pressure may provide less than half the die retention force needed. In such cases, dies have become detached from the press with catastrophic results. Another common error to avoid is tampering with the pressure safety switch. This is sometimes done to permit the press to run with less than the required hydraulic clamp pressure. Avoiding a production delay due to an oil leak is an unacceptable motive. However, if the die becomes detached, the damage and production delays can be extremely expensive. Good engineering and strict maintenance procedures are required for safe dependable operation of hydraulic clamping systems. An essential safety feature of hydraulic clamping systems is that a loss of retaining force will shut down the press. This is not the case with mechanical bolting systems. If clamping force is marginal, the pressure in the clamp circuits can intensify. This can be detected as a pressure pulsation on the hydraulic lines to the clamps.

Figure 2. End view of a ledge type hydraulic clamp. Design for drawing courtesy of Hilma Division of Carr-Lane Roemheld. Smith & Associates

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Hydraulic Ledge Clamps Figure 2 illustrates an end-view of a type of hydraulic ledge clamp. This type of clamp is popular for both new and retrofit applications. The clamp body and user-supplied spacer block are fastened directly to the press ram or bolster by capscrews. Up to six or more individual spring-return pistons are available in this design. The pistons are supplied with hydraulic pressure by means of internal drilled passages. One advantage of this clamp is that the drilled passages lessen the need for external hoses and piping required, thus eliminating potential sources of leaks. For additional safety, this design can be supplied as a split system supplied by two individual pressure sources. Some designs are supplied with hydraulically controlled check valves that require a separate hydraulic pressure to release the clamp. The hydraulic ports are shown in the clamp body. The clamping surface on the die shoe or sub-plate must be of a standard height to permit interchangeability of different dies. The standard clamping height chosen determines the thickness of the user-supplied spacer block. Typically, 0.5-inch (12.7-mm) clearance is provided between the retracted pistons and the die-clamping surface. Each application requires individual evaluation. Ledge clamps that are bolted in place must provide ample piston travel clearance in excess of the lifting stroke if bolster roller lifters are used.

Figure 3. A hydraulic T-slot clamp. This type of clamp is popular for retrofit applications. The height of the die shoe or surface clamped must be a constant dimension to permit interchangeability of tooling and the correct amount of piston travel. Design for drawing courtesy of Hilma Division of Carr-Lane Roemheld. Smith & Associates

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Sliding Clamp for Use in a Conventional T-Slot Figure 3 illustrates a type of clamp that is popular for retrofitting existing presses for hydraulic clamping. It has several noteworthy features that include:

1. It is easily installed in existing T-slots without bolster or ram modifications. 2. It clamps over a plate or ledge—U-slots in the die shoe or sub-plate are not

required. 3. Clamps of this general design are available from several sources. 4. It is well suited to most light duty short run jobs.

Example of Sliding T-slot Hydraulic Clamp The sliding T-slot hydraulic clamp illustrated in Figure 3 is a good example of this style of clamp. The T-slot adapter is available for several standard types of T-slots. The standard clamping height chosen determines the finished height of the user-supplied spacer block. Typically, 0.157-inch (4-mm) clearance is provided between the retracted pistons and the die-clamping surface. The clamp body is attached to the T-slot adapter with screws. The hydraulic piston is a single-acting spring-return type. Hydraulic pressure is applied through a standard port fitting. An optional design employs a second port and a hydraulically controlled check valve to prevent the release of holding force in case of a line failure. The clamping surface on the die shoe or sub-plate must be of a standard height in order to permit interchangeability of dies using this type of clamp.

Specialized Pull in Type Clamps Figure 4 shows a double-acting pull-in clamp installed in a press bolster. It may also be installed in the press ram by inverting the clamp. In its simplest form, this type of clamp has an up and down motion for unclamping and clamping in a slot cut in a die shoe or sub-plate. It is shown in the unclamped position in a tee slot cut in a die shoe. In this position, the die shoe can be slid in or out of either side of the press. Design of a Simple Double Acting Pull in Clamp The cylinder body as shown in Figure 4 is fastened into a bored and countersunk hole in the bolster and/or press ram. Proximity switches inside the cylinder sense the clamp position travel and provides an electrical signal by means of the connector on the bottom of the cylinder. This is a safety feature and can be interlocked with the press controls to permit proper sequencing of die movement for automatic die changes. Depending on operating pressures, the number of clamps used and piston area, this style of clamp can be used for a wide range of die weights and clamping requirements.

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Spring Applied Pull in Clamps For light duty die fastening requirements, the pull in clamp can be applied by spring pressure. A compact type of spring contains a stack of dish shaped Bellville spring washers to supply the holding force.

Figure 4. A pull-in type clamp installed in a press bolster. Design for drawing courtesy of Hilma Division of Carr-Lane Roemheld. Smith & Associates Swung-Sink Pull-in Clamps In addition to the up-down movement of the pull type clamp, shown in Figure 4, other types of clamp movement can be achieved in the same basic type of clamp body. By the addition of an internal helix mechanism, and additional proximity switches, the head can be made to lift and swing 90 degrees, and then sink below the bolster surface. These features permit the clamp to engage a slot, or in the case of the swing-sink clamp, engage an elongated hole in a die shoe or sub-plate. Here again, rigorous preventive maintenance must be performed to assure that the die is properly located so the clamp does not hang up in the locked position with the die in the press. A potential danger is that the clamp may fail to release. Should this occur, a procedure approved by the manufacturer should be followed. Any overriding of the position sensing function requires care. Where possible, access to the clamp should be provided in the die and bolster design.

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A swing sink clamp of modern design seldom causes problems. Proximity sensors monitor the correct sequencing action of the clamping and unclamping. The clamps are operated by programmable logic controllers that monitor the clamp and unclamp position of each clamp. Should one clamp fail to release, the press cannot be inched. This system should never be tampered with to render any self-checking function inoperative.

Automatic Traveling Die Clamps Figure 5 illustrates a complex automatic die clamping system attached to a press slide. An electrical motor with a gearhead drives the mechanism that positions the clamp. With the die in the press, and the press closed, the motor drives a flexible track with a clamp on the end into position to clamp the upper die in the press. 1 This system is expensive and somewhat complex when compared to a clamp that simply slides or swings into position. However, it permits automatic die change in transfer presses and tandem lines without the extra expense and weight of an adapter plate the width of the press ram.

Figure 5. An automatic traveling die clamp attached to a press slide of a large transfer press. Auto Alliance International

1 D. Smith, Quick Die Change Video Course, Tape one, session four has animated footage of how the motorized traveling clamping head works. The case study is from Auto Alliance International, Flat Rock, Michigan. The workbook has a description of the clamp on pages 46 to 47. The same description is in the Facilitator’s Guide on page 73 to 74.

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Appropriate Traveling Clamp Applications This type of clamp is much more complex and expensive that our earlier examples. The most cost-effective applications involve difficulty to hand position the clamp on large presses. Here, diesetter safety alone can justify the cost of the system. It is especially effective on large tandem and transfer presses having moving bolsters. It is essential to have a locating pin system position the die clamping slots in alignment with the traveling clamps. Again, the purpose of the traveling clamp is to avoid the need of a subplate the width of the press ram for each die. The traveling clamp will engage and secure milled pockets in the upper die shoes. Design Variations and Power Sources The example shown uses an electric motor and a gear reduction drive with a sprocket system driving a rigid chain to move the clamp. Other positioning systems may use lead screws or belt drive methods. Designs use hydraulic or electric clamping. The power source to move the clamp into place may be an electrical motor, air motor, hydraulic cylinder or air cylinder.

Hydraulic Clamp Application Examples American Yard Products (AYP) leading manufacturers of both push type and riding lawn care products. One of the manufacturing plants located in Orangeburg, South Carolina is the basis for the following case study. 2

AYP has made major diesetting improvements through standardization of die location, permanently attached parallel application to permit simple diesetting with fork lift trucks and the use of standardized application of hydraulic die clamping. These improvements together with die storage near the point of use, layout changes to organize stamping work cells and rapid coil change provisions have continually increased plant efficiency and throughput. Clamp Standardization The sucessful use of hydraulic clamps at AYP requires an ongoing program of building new tooling with standard clamp ledges as well as adapting existing tooling to provide a constant clamping ledge or subplate thickness. Whenever conversions are in process to convert from manual bolting to hydraulic clamping, a means to assure safe operation that includes proper storage of unused hydraulic clamps must be provided.

2 Mark Turcich, Using Simple Solutions to Implement Quick Die Change, The Stamping Quarterly, The Fabricator's and Manufacturer's Association, International, Rockford, Illinois, May-June © 1996.

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Figure 6. Example of a hydraulic clamp sliding into a T-slot. This type of clamp features a high force capacity up-acting cylinder that actuates a pivoting clamping bar. The armor protects the hydraulic hose. American Yard Products / Kosmek USA Ltd. Figure 6 is an example of the type of clamp that American Yard Products has standardized for many stamping operations. The clamp is sliding into a T-slot. Standardizing on one or a limited number of power clamps is highly recommended. Advantages of Hydraulic Clamp Standardization Standardizing on only one or a limited number of clamp styles has many advantages including:

1. Standardized clamping height for die shoe clamping surfaces, parallel ledges, sub

plates etc. will save time, and avoid errors. 2. Employee training is simplified if only one or a limited number of power clamp

types are used. 3. If more than one type of power clamp is used, they should not be similar in design

and function. This will help avoid mixing types accidentally. 4. Stocking of spare parts, periodic preventative maintenance and repair procedures

are simplified. This will help avoid downtime and minimize repair costs. Robust Construction is Necessary Figure 7 illustrates a clamp slid into position and actuated. A feature of good hydraulic clamping systems is robust construction. This design features a wide heavy pivoting bar and large diameter pivot pin.

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Figure 7. The clamp shown in Figure 6 slid into position and actuated. Note the wide heavy pivoting bar and large diameter pivot pin. Robust construction is required for long service life and safe die retention. American Yard Products / Kosmek USA Ltd.

Figure 8. Hydraulic oil reservoir having a sight glass to indicate oil level. An air actuated hydraulic pump is over the reservoir. Note the air supply filter, lubricator and pressure regulator that are attached to the air over oil intensifier pump. American Yard Products / Kosmek USA Ltd.

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Robust construction is a feature of any dependable die clamping system if long service life and safe die retention is to be achieved. The die fastening system must withstand a variety of pressworking lubricants, slivers, slugs and mechanical impact from parts and scrap.

Any properly designed die fastening system must retain the die safely and securely in the press. Powered die-clamping systems feature automatic press shut down in case of a loss of holding force. Hydraulic Pressure Source The most common source of hydraulic pressure for die fastening applications is air over oil pump system. The pump or intensifier operates by means of two reciprocating pistons of different diameters. The air-actuated piston has a much larger area than the hydraulic piston. The ratio of the piston areas and applied air pressure determine the approximate hydraulic pressure produced. The reciprocating action is accomplished by means of mechanically actuated air reversing valves to route compressed air to the air end of the intensifier and check valves on the hydraulic end of the intensifier pump.

Figure 9. View of the hydraulic pressure source valves, pressure safety switches and steel tubing hydraulic lines installed on the side of a straightside press. American Yard Products / Kosmek USA Ltd. The main advantages of air over oil intensifiers are simplicity and economy. Once the pressure is applied to a leak free clamping system, there is no air consumption. The intensifier pump is shown Figures 8 and 9 mounted on the top of the oil reservoir. The air supply filter, lubricator and pressure regulators are also shown in these illustrations. Note that the correct air pressure is marked on the air pressure gauge.

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Oil Reservoir Care The oil reservoir is usually a simple cylindrical or rectangular tank that stores hydraulic oil at atmospheric pressure. A sight glass as shown in Figure 8 indicates the amount of oil in the reservoir. When the clamps are actuated, oil is pumped from the reservoir into the actuating pistons of the clamps. A slight drop in the oil level in the reservoir is normal. The oil is returned to the reservoir when the clamps are released. It is important to use the correct type of hydraulic oil specified by the system supplier. Use of incorrect oil can damage the seals and other components of the system. Slight oil seepage from seals is normal in many hydraulic systems. Over time, the oil level can drop slightly and need the correct hydraulic oil to be added to maintain the correct level. Any excessive consumption should be noted by the press operators, diesetters and maintenance technicians. Excessive oil loss indicates leakage that must be repaired promptly.

Figure 10. An unused lower die clamp should not be dropped onto the floor. Both the clamp and especially the hydraulic hose may be damaged. This can result in system leaks and malfunction. American Yard Products / Kosmek USA Ltd. Hydraulic Die Clamping System Maintenance If normal care is practiced and the system manufacturer’s instructions followed, a powered die clamping system should require a minimal amount of periodic maintenance. The high-pressure hoses and connections tend to be the highest maintenance items. This is because of exposure to both repeated flexing from die changing as well as damage from a variety of pressworking lubricants, slivers and slugs.

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Special attention must be given to inspecting the level of fluid in the reservoir to determine if leakage is occurring. The recommended hydraulic fluid should be used. It may be necessary to rebuild the clamps after extended use. Any worn parts must be replaced with new parts in accordance with the manufacturer’s instructions.

Figure 11. An unused upper die clamp should not be stored on the ram ledge. The clamp can fall off endangering pressroom personnel. In addition, the clamp, and especially the hydraulic hose may be damaged. American Yard Products / Kosmek USA Ltd.

Figure 12. Storage brackets used to hold unused hydraulic clamps. This feature is used during die change and in the event that a die not adapted for use with the clamps must be set with manual bolting methods. American Yard Products / Kosmek USA Ltd.

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To assure safe operation, the function of the low-pressure switches must be carefully set and maintained. The function of these pressure-sensing devices is to stop the press in case of a drop in pressure below the value required to retain the die in the press safely. Comparing Powered Systems to Manual Bolting If good engineering and cost accounting practices are followed, powered clamping systems are both safe and can be cost effective. Unlike mechanical bolting systems, the low-pressure alarm feature will stop the press in case of a drop in pressure. Conversion to Powered Clamping Figure 12 shows brackets used to hold unused hydraulic clamps. This feature avoids placement of the upper die clamps on the ram ledge and floor as shown in Figures 10 and 11. A system to securely store unused clamps while existing dies are being adapted for use with the clamps must be provided. During the retrofitting period, old dies may require setting with manual bolting methods. NOTES: _____________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________

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NOTES: _____________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________

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power actuated die clamps - smith & · pdf filepower sources used to actuate die clamps...

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