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Blanking & Piercing operations can create reverse snap thru tonnage which will damage you press and tooling. With applications of this nature, hydraulic punching dampers can be added to reduce reverse snap thru tonnage. A good rule of thumb, if reverse tonnage is over 10% of press capacity, add dampers.

In this multipart topic on Reverse Tonnage also referred to as “Snap-Through” we will examine the root cause of this issue and its adverse effects on both your press and your tooling. The effects of Reverse Tonnage can be devastating. If not addressed properly over time Reverse Tonnage will literally reduce the life of your tooling and destroy the drive train of your press. The results of ignoring Reverse Tonnage can mean a complete rebuild of your press which can be hugely expensive.  However, today Reverse Tonnage is a well understood side effect of performing “blanking” in a press and its harmful effects can be controlled.

Snap-Through

Snap Through – What is it? Somewhere during the rotation cycle of your press prior to reaching bottom dead center your tooling engages the surface of your material.  As the rotation cycle progresses over the course of microseconds an immense amount of energy accumulates in the drive train of your press and the tooling itself. This is due to the resistance of the material to being pierced by your tooling. The stored energy accumulates until it reaches a point sufficient to overcome the resistance of the material.  Here in lies the problem. In an instant all that stored energy is released as the tooling pierces or “Snaps- Through” your material. This instantaneous and uncontrolled release of energy sends a shock wave through your entire press.  Also keep in mind the greater the area to be pierced or the thicker or higher strength your material is the greater amount of energy is stored and released.

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Reverse Tonnage

Reverse Tonnage – What does it effect and why?  The Drive Train of your press is designed to deliver the working energy from the motor to the tooling in one direction – Forward. The Drive Train is comprised of several components: Gears, a Drive Shaft (Crank Shaft), Bushings, Tie Rod(s), and Ball Seat(s). In order to work properly all these components must have pre-engineered clearances.  This small amount of gap between the components allows the different metal surfaces of each component to slide along one another.

During “Snap Through” the clearances between the individual components will move abruptly and with great force from one side of their connection to the other.  For example the Tie Rod(s) are connected to the Crank Shaft(s) with a Bronze Bushing(s). The Bronze Bushing is perfectly round and it’s inside diameter is slightly greater than the outside diameter of the Crank Shaft.  During the downward stroke cycle when the working energy is being delivered to the tooling the bottom side of the Crank Shaft and the Bronze Bushing come into direct contact with each other.  All the clearance is driven to the top of the connection.  

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This is the proper working cycle of the drive train. However, during “Snap-Through” the sudden release of the accumulated energy discussed earlier causes the Tie Rod(s) with the Bronze Bushing to lurch downward. When the Tie Rods lurch downward the Bronze Bushing slams into the topside of the Crank Shaft(s). The clearance in the connection point is reversed from the top of the connection to the bottom. Hence the term “Reverse Tonnage”. This same clearance reversal happens throughout the entire drive train. This sudden and uncontrolled release of energy sends a shock wave through your press and tooling. The drive train of your press must absorb the brute of this shock wave with every stroke. Over time, this uncontrolled release of energy will cause the round Bronze Bushing to become “Egg Shaped”. In addition the rest of the drive train will also have excessive wear and damage requiring in some cases a complete rebuild of the press.

In Part 2 of this series, we will examine ways to control Reverse Tonnage and its effects on your equipment.

In the last section we examined the cause and effects of Snap Through and Reverse Tonnage. While Reverse Tonnage is a fact of life today it is well understood and it adverse effects can be controlled. In this issue we will examine how much Reverse Tonnage is acceptable. How Reverse Tonnage can be measured and the options available to you to address the effects of Reverse Tonnage.

Reverse Tonnage – How much is acceptable?

We all know Reverse Tonnage is a fact of life we must all deal with in metal forming. However, today's modern press designs take this in to account. Today's modern press designs can typically tolerate up to 10% of its total tonnage in Reverse Tonnage / capacity with no adverse effects.  For example if you have a 100 ton press it should be able to tolerate 10 tons of Reverse Tonnage with no ill effects on the press. This amount of Reverse Tonnage should be tolerable over the lifetime of the press. It is when a press routinely encounters Reverse Tonnage above this 10% margin that troubles begin.

How do I measure Reverse Tonnage?

Today there are advanced control systems available which can measure Reverse Tonnage for you and display it on a screen. These advanced systems utilize strain gauges attached to the frame of the press in various areas depending on the design of your press. These measurements are very accurate and the control system can provide you a readout of reverse tonnage with every stroke of the press in real time. Some of the most advanced control systems can also display the amount of working tonnage encountered by different areas on the press.  For example the display shown here from I-PRESS® is for a Mechanical Straight Side Press. The display shows Reverse Tonnage as well as the amount of tonnage exerted on the four corners of the press. The most advanced control systems will constantly monitor the amount of Reverse Tonnage the press encounters as well as monitor the amount of working tonnage exerted on the four corners of the press.  

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With these highly advanced systems you can set a high and low tonnage setting for each corner of the press. These control systems then monitor the tonnage and stop the press if the measured tonnage falls outside of your preset parameters. For example you may have slugs building up in your tooling so you will exceed the high setting or perhaps a punch has broken off in your tooling and low you will exceed your low tonnage setting. This type of constant and accurate monitoring can catch many problems as soon as they start. In turn allowing you to head off any additional problems that could be caused and address the issue at hand quickly for reduced down time.    

So now we know what Reverse Tonnage is. We know its cause and ill effects. We know how much is acceptable and how it can be measured. So what solutions are available to control Reverse Tonnage?

Press Size

You could consider over sizing your press based on the jobs you process. For instance if you had a 200 ton press processing jobs typically processed on a 100 ton press your 10% margin would be 20 tons instead of 10 tons. This is because the larger your press the more mass you have which can absorb the Reverse Tonnage.  While this is an option, it does not make a lot of economic sense and would be cost prohibitive.  There are other more economical ways to address Reverse Tonnage.  

Tooling

Your first line of defense against Reverse Tonnage is your tooling.  As mentioned in our last issue, you have the instantaneous effect of Snap Through and Reverse Tonnage.  However, with proper forethought and designing of your tooling Snap Through and Reverse Tonnage can be minimized. Consider if all the punches in your tooling are the same height. Punches are the elements of the tooling which pass completely through your material. When all the punches are the same height they will all Snap Through your material at the same instant. This tooling design places the greatest amount of Reverse Tonnage on your press as is possible with the job at hand. This is why it is always important to evaluate the design of your tooling.  As much as possible stagger the height of the punches in your tooling.

By staggering the height of the punches they complete their tasks in succession and not all Snap Through the material at the same instant. This minimizes Reverse Tonnage because, as one set of punches Snaps Through the material another set of punches are beginning to enter the material there by offsetting the Reverse Tonnage. This is a simple and very effective method of addressing Reverse Tonnage. However, it is many times over looked.  If staggering the height of the punches keeps your Reverse Tonnage under the 10% margin discussed earlier - Problem Solved.  

Hydraulic Shock Dampeners

When utilizing large complex tooling or sometimes due to the job at hand it may not always be possible to reduce Reverse Tonnage under the 10% margin discussed earlier.  In these situations your next line of defense are Hydraulic Shock Dampeners. These are separate self-contained hydraulic devices which work much like the shock absorbers on your car. Hydraulic Shock Dampeners are typically retrofittable onto both new and used presses. Hydraulic Shock Dampeners are always used in a set of 2, 4 or more depending on the size of your press.  When two are used they are placed on the right and left hand side of your press and centered front to back on the bolster. When four are used they are set on the four corners of the bolster.  

You must always use Hydraulic Shock Dampeners in sets of two to ensure the load is centered on your press. The dampeners height is adjustable so it can be set to come in contact with the presses slide at the same moment your tooling Snaps Through the material. The Hydraulic Shock Dampeners are designed to provide counter balance force against the slide to absorb the Reverse Tonnage energy at the moment the tooling Snaps Through the material there by greatly reducing Reverse Tonnage to very tolerable levels.

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With these very desirable results why would you not always use Hydraulic Shock Dampeners? The only potential drawback is they take up precious real estate in your bolster area.  Sometimes due to the size of your tooling there is not enough room left on your bolster to use Hydraulic Shock Dampeners. However, some press manufacturers can offer innovative press designs where the Hydraulic Shock Dampeners are incorporated into the side frames of the press. This innovative design permanently eliminates the need to place the Hydraulic Shock Dampeners on the bolster. The novel design approach allows for all the benefits of the Hydraulic Shock Dampeners without giving up any precious bolster space.

In the end Reverse Tonnage is a fact of life we all have to deal with every day. However, its ill effects on your press and tooling are well understood.  As we have seen there are different ways to manage and control Reverse Tonnage depending on your circumstances.  Which method is best is really a team effort between you, your tool maker and your press supplier.  Just be sure to always address Reverse Tonnage so You are not Beating your Press to Death.

Your press in many ways is absolutely vital to the success of your business. The thump, thump, thump of your stamping press is the heart beat of your company. With every thump another part is made and your business prospers. To keep that heart beat strong your operators need to be sure to make the proper setups and adjustments on your stamping press as required for each job. The Air Counter Balance is one of the easiest adjustments to make on your press. Yet this very important system adjustment is many times over looked to the long term detriment of your mechanical press. If overlooked for too long the cumulative damage caused by not properly adjusting the Air Counter Balance (ACB) for each job can be so severe it can  require a complete rebuild of the drive train of your stamping press. This is a very costly repair that is easily avoided.

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A typical Air Counter Balance System will incorporate an Air Cylinder(s), Air Regulator, Pressure Gauge, Air Dryer with Filter, System Decompression Valve, and a Compressed Air Tank (Reservoir) with Drain Plug and an Over Pressurization Relief Valve.  Depending on the size of your press the Air Counter Balance System will incorporate one or two Air Cylinders.  If there are two cylinders, one cylinder is located on each side of the press left and right. Compressed air to power the system is provided by an outside source.  

When in operation starting at Bottom Dead Center of the rotation cycle air from the air reservoir is pumped into the bottom of the air cylinder. The ram (picture right) of the air cylinder is connected to the upper slide of the press which also holds the upper die tooling. When in operation and adjusted properly a metered amount of compressed air fills the air cylinder at a specific pressure and rate. This in turn drives the cylinder ram upward at the same speed as the rotation cycle of the press. The Air Counter Balance System will lift the combined weight of the upper slide and the upper die tooling for the drive train of the press. Once the press has reached Top Dead Center the compressed air is released from the air cylinder(s) at a metered rate that will match rotation speed of the press.  This will keep back pressure against the upper slide and drive train. This keeps all the connections in the drive train in a (compressed) state.

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Why does a Press need an Air Counter Balance System?

The rotation cycle of a mechanical press can be separated into two distinct halves: the Downward Stroke (Past TDC approaching Bottom Dead Center - Compression) and an Upward Stroke (Past BDC returning to Top Dead Center - Tension). All the connections in the drive train of your stamping press are designed with small clearances in them to allow the metal surfaces to slide passed each other. During the Downward Stroke of the rotation cycle the connections of the drive train press against one another in the same direction to move the tooling forward to complete its work. This drives all the small clearances to one side of all the connections throughout the entire drive train. Think of the drive train as being under compression pushing forward to deliver the working energy to the tooling. However, once the press has reached BDC and the Upward Stroke begins the workload now goes in reverse (Tension).    

During the Upward Stroke the drive train must lift or pull the Upper Slide along with the Upper Die Tooling to TDC. The amount of weight to be lifted can be significant. Now the drive train is under Tension.  All the forces in the drive train are reversed. Instead of pushing forward (compression) to deliver working energy to the tooling the drive train is now pulling (tension) to lift the upper slide and upper tooling. Without an Air Counter Balance this reversal in workload will cause all the small clearances in the connections of the drive train to instantaneously move to the other side of the connection.  

Over time without the Air Counter Balance being properly set will have the same devastating effect on the drive train as Reverse Tonnage. By allowing the constant uncontrolled reversing of the work load on the connections in the drive train with every stroke of the press will damage the connection points and will over time require a major rebuild of the drive train.  In short the Air Counter Balance will prevent the reversal of the workload keeping the drive train under compression there by preventing the engineered clearances in the drive train from moving back and forth.  

We now know what an Air Counter Balance System is, what the system does and why it is important to properly adjust this vital system for every job processed. In the next issue of Press On and Forge Ahead we will examine how to properly set the Air Counter Balance and proper maintenance of this system.