Bend within limits to avoid trouble
January 10, 2006
Every press brake is subject to normal deflection under load. This deflection is corrected by shimming. If you deflect behond design limits, you will put a permanent bend in the ram and this is known as ram upset. You cannot adjust to compensate for ram upset. Remachining of the ram is the only solution. To avoid causing ram upset be careful about bending loads above your tons per inch limit and only air bend if possible.
Of all the potential problems you might have when bottom bending or coining with press brakes, the least understood is a problem called ram upset.
Ram upset is a condition in which the press brake ram is permanently deflected in the vertical plane, and so the distance between the ram and the bed is greater in the center of the machine than at the ends. As a result, long parts are bent with a tighter angle at the ends than at the center. For example, if you bend a part 4 feet long, you might end up with a 90-degree bend at the ends and a 93-degree bend in the center. This is referred to as a canoe-shaped part (see Figure 1).
Bed and ram upset will cause parts to become canoe-shaped.
What Causes It?
Ram upset also can be caused by an excess of ram deflection, which is a normal press brake action. Any beam that is supported at each end (for example, a bed and ram) and is subject to a load deflects. This is why press brakes are crowned, usually in the die holder or in the bed.
Some types of press brakes are crowned in both the bed and ram. The amount of crowning usually is 0.0015 in. per foot for half the distance between the side frames. For example, a machine with 10 ft. between the side frames would have 0.0075-in. crowning [0.0015 in. x (10 x 1/2)]. The crowning partially offsets the deflection that naturally occurs under load.
When a load is so concentrated that the ram deflects beyond its design limit, and the amount of deflection is more than the crowning, the ram will "take a set," or upset. When ram upset happens, shimming must be used in addition to crowning to counter the ram deflection.
Normal, or designed, deflection is based on the thickness and depth of the press brake ram as well as the bed. Normal deflection is the amount of deflection that the ram and bed can be subjected to and still return to a stable condition after the load is removed. It is when the load deflects the ram and bed beyond the design limit that ram upset occurs.
To avoid overdeflection of the ram, do not load the press brake at full capacity over less than 60 percent of the distance between the side frames.
The nature of air bending is such that you cannot exceed the normal deflection limits. This means that when you are air bending, your machine can and will deflect within normal limits, but you cannot apply sufficient tonnage to create ram or bed upset. Therefore, you should use air bending whenever possible.
The upset usually occurs in the ram because the load is more concentrated on the ram. The load is more evenly distributed on the bed than on the ram because of the die rail and the lower tooling.
You should find out how much deflection each load creates so that you can shim the bed accordingly. This is why you must estimate the load required.
Most press brakes are designed for maximum allowable deflection when a full-tonnage load is applied uniformly over 60 percent of the distance between the side frames (see Figure 2). This means that a 100-ton press brake with 10 ft. between housings will deflect to design limits with the 100 tons applied over 72 in. in the center of the machine. If the 100 tons is distributed over a distance shorter than 72 in., the ram will exceed design deflection limits and will upset.
The design limit for deflection is 0.0015 in. per ft. for the distance between the side frames. Therefore, for a press brake with 10 ft. between the side frames, the allowable deflection limit is 0.015 in. for the bed and 0.015 in. for the ram. The deflection calculations for tonnage are based on bottom bending or coining.
To be on the safe side, you should calculate a forming limit in tons per inch for the brake. Make sure that you do not exceed that limit with each part to be formed. For example, for the previously mentioned machine with 10 ft. between housings and 100 tons distributed over 72 in., the maximum tonnage limit would be 1.4 tons per inch.
With the help of two other people, you can check your press brakes for ram upset using a piano wire or a 50-pound test fishing line. You and another person hold the wire under (behind the tooling) and at each end of the ram. A third person in the center at the back of the brake and using a feeler gauge measures the distance from the ram surface to the wire in the middle of the machine. Be careful not to apply so much thickness of feeler gauge that you move the wire.
Be sure that the wire is pulled very tight while measuring. Also, don't forget to turn off the power and to wear gloves.
The best way to compensate for deflection is to shim the bed or use a bed deflection compensation device if available.
The maximum allowable ram upset is 0.015 in. Upset more than 0.015 in. would require that the bed and ram be remachined. This should be done professionally. You will have a difficult time forming straight parts on an upset bed and ram.
In summary, every press brake is subject to normal deflection under load. This deflection is corrected by shimming. If you deflect beyond design limits, you will put a permanent bend in the ram—ram upset. You cannot make adjustments to compensate for ram upset. The only recourse is to have the bed and ram remachined. To prevent ram upset, be careful about bending loads above your tons-per-inch limit, and air bend when possible.
Bob Butchart is the owner of Press Brake & Shear Clinic, 419 Lakewood Drive, Lexington, NC 27292, 336-248-5881, fax 336-224-6665, firstname.lastname@example.org.