Running a top-notch, efficient shop means using the proper pulling techniques
The basics
Modern vehicles absorb a great deal of energy from an impact. That impact energy dissipates throughout the structure dimensionally, creating what's called a "cone effect." From a relatively small impact area, the damage expands through the rest of the body like a cone to disperse energy and thereby protect vehicle occupants.
In the past, with conventional heavy frames it was possible to use a very limited number of pulls in a single direction. Repairing third-generation unibody vehicles is completely different. The challenge here is that the point of impact area normally is not strong enough to pull all the (dissipated) damage into alignment unless the damage is very minor.In the past, shops used shear force to "overpower" the frame. Today using that same force can tear panels apart. Finesse, not force is the proper approach. Proper anchoring, blocking, pulling and stress relieving is vitally important for repairing these cars. Shops must take four actions for a proper pull:
1. Have the vehicle properly anchored and blocked;
2. Ensure the direction of pulls reverses the direction of the area moved;
3. Accurately maintain control of the pulling force;
4. Once the pulls are made, measure the progress, relieve the stress area and repeat.
Anchoring
Whatever pulling force is placed on the vehicle is transmitted to the areas of anchoring. The anchoring area must be secure enough to prevent vehicle movement and also be able to withstand the pulling force without distorting the anchor area. On most unitized structures, the anchoring area will be the pinchweld area at the front and rear torque box since this area generally has some reinforcements and/or multiple layers of metal.
Some vehicles don't have reinforced pinchweld areas or a pinchweld at all. These vehicles either have special anchor adapters designed for the pulling system or require modifications to the structure for anchoring. Modifications can include welding or removing suspension parts to bolt the anchoring fixture. Never vary from the recommended anchoring locations or make your own.
You also shouldn't restrict yourself to a limited number of pinchweld clamps. Figure 1 demonstrates how the pulling force is divided by the number of anchor points. In this example, two clamps exert 1,000 lbs. of pressure at the anchoring points, which will distort and damage the mounting areas.For most pulling systems, each pinchweld clamp has multiple smaller "jaws" attached to it. These divide the pulling force at each clamping location and help prevent pulling damage. Figure 1 illustrates how a four-clamp system with two smaller jaws on each clamp reduces the pulling force on each anchor point. Even with 2,000 lbs. of pulling force, the maximum stress on each pinchweld anchoring point is 250 lbs. (We arrive at this number by dividing the 2,000 lbs. of pulling force by eight jaws — two per clamp unit.)
You can determine the actual pounds per square inch on each area by factoring in the actual jaw face's surface area. The vehicle manufacturer's engineers and equipment engineers analyze these factors to make sure the mounting area can withstand normal pulling force if properly mounted. In some cases, if the area can't withstand these forces, the engineers will provide alternatives necessary to properly anchor the vehicle.
You must properly mount the vehicle. If the pinchweld clamps are not properly tightened and the clamp jaws slip, force becomes directed to the other mounted clamp jaws. It's very important to ensure the pinchweld clamps are free from any obstructions that might interfere with secure anchoring. Pinchweld clamp jaws should be inspected, cleaned regularly and replaced periodically when they become worn or damaged.When working on frame type vehicles, use a multiple sized frame hole clamp. Always keep this clamp horizontal during a pull. Do this by placing blocks under the clamp so it cannot be pulled down at the mounting hole. You also can place a small hydraulic ram on the clamp.
Blocking
Blocking involves using leverage to ensure pulling force is concentrated in the proper area of damage. Blocks are placed (normally upward) at a damaged area needing upward movement. They also can keep an area from moving downward during the pulling process of another area.
As with anchoring clamps, the force placed by blocks on a location is determined by the amount of force placed against it during pulling. A block placed under a cross member or structural member exerts the same amount of force upwards as the pull is directing downward. This concept is useful when restoring datum misalignment.During pulling, some of the pulling force is absorbed in the body due to the flexing of the unitized structure. The additional force necessary to create movement in the damage area can tear the pulling clamp areas. This is analogous to breaking a green tree branch versus an older, brown one. The green branch requires more effort to break because it flexes before breaking. Blocking can assist in lessening the flexing.
Holding
Holding fixtures too can assist in eliminating body flex. One common type of holding fixture is a turn buckle. This unit has small clamps on each end that can be mounted in a door opening and tightened to reduce upper body flex.
Direction of pulls
Pull direction is sometimes misunderstood. Note that just because the collision impact was at the vehicle front doesn't mean all the pulls need to be directed forward. There are many designed bends in a structural member that deflect collision energy in a direction other than straight backwards. These bends usually are found in secondary damage areas and can be, at times, difficult to move during initial pulling.
The term "multiple pulls" similarly is misunderstood. Multiple pulls involves, in part, dividing up the pulling force into more than one area (that probably would tear by itself). It also involves pulling in the direction the damage needs to move.There are frequent occasions when an impact moves an area in a direction unlike the primary impact direction. This can occur during a frontal collision when the energy travels up through the A-pillar (windshield post) and causes the roof panel to buckle at the B-pillar (door center post). This buckle doesn't result from the roof panel moving backwards (primary frontal impact) but from the front portion of the roof moving upward from the A-pillar. Because the roof is anchored with the B-pillar, it becomes a hinge point, hence the buckling. If pulls were made only in the opposite direction of the primary damage, these areas would have a difficult time returning. These areas must be moved with pulls or blocks in the right direction.
Pulling only from the front generally will not restore the roof to its proper location. The turn buckle can be used to pull the front corner of the roof down and hold it into place during the rest of the repairs, lessening body flex and removing the buckle stress. Using proper directional pulls in secondary damage areas and blocking/holding will dramatically reduce the amount of pressure needed to pull vehicle structure.
Controlling the force
Unitized structures should be pulled with a minimal amount of force. How do you know what level of force is minimal, especially if you don't have a gauge to monitor applied pressure? Counting the seconds between each pump beat is not recommended since it's not accurate enough. You must be able to exactly measure the pressure each of your pulls places on the structure to verify the pulling plan is the correct action.
If you have to use excessive force (usually anything over 2,500 to 3,000 lbs.), your pulling plan is probably not correct. There may be times when pulling a reinforced center section area that this suggested limit is reached, but I've found staying below 2,500 lbs. is an achievable goal.
Many of today's pulling systems have pressure gauges for this purpose, though they do present some challenges. If the pulling system uses foot hydraulic air pumps, these gauges have a tendency to become damaged since they often are mounted at the pump base. Some gauges are located on the frame rack itself, making them difficult to see during a pull. Still, you always should use these gauges to properly monitor pulling force.
You can reduce the amount of force needed to pull today's vehicles by identifying any obstruction that may be restricting the damage from moving. Obstructions can include the engine cradle or cross member. You may need to unbolt the front bolt of a damaged engine cradle to allow for the rest of the structure to move. If the rear engine cradle mounting area is out-of-specs then a pull can be added to the cradle itself to pull that location while the other structure is being pulled. It also may help to cut away some of the primary damage to allow access to the rest of the structural damage.
The pulling process
I've been in many shops where the clamp assortment is sparse and where bolts for each clamp are shared because they have not been maintained. This should never happen. Make sure the clamp bolts are replaced regularly and numerous clamps are available. It is not efficient to pull, stop, change clamps to another location, and then pull again.
Once everything is properly anchored, blocked, and the clamps are mounted, you may begin pulling. Pulling and continuing to pull without relieving the internal metal's state typically will cause the structure to crack or tear. Metal becomes work-hardened where it is damaged. Stress relieving of the metal around the damage is necessary.
Stress relieving is accomplished with vibration (heat or mechanical). There is some misunderstanding surrounding the use of heat that has led to many organizations not recommending it. The heat used for stress relieving warms the molecules up to allow them to internally vibrate (to move). The heat isn't intended to "soften" the steel and is very low.
Mechanically creating vibration is easy as well as the most effective method. It can be accomplished with hammering spoons or pneumatic bits. The vibration quickly reduces the metal stress and reduces the pulling force needed. If a pull is held at 1,500 to 2,000 lbs. and the pulling direction is correct, during stress relieving (vibration) the pressure gauge should drop significantly during the hammering.
Avoid hammering directly on a buckle until it is ready to move and be straightened. Hammering directly on a buckle can harden it further and make it almost impossible to straighten.
This process of pulling to a reasonable pressure, holding, stress relieving the area and repeating the process produces the most efficient pull process. It also prevents the tearing or cracking of metal.
Keep in mind, though, that if the material was stressed past its recommended point, it may already have micro fractures and need to be replaced anyway.
The new substrates
With the new metals, namely aluminum and high strength steel (HSS) used in vehicles, new pulling rules apply. The new steels currently include dual phase (DP) steels of various categories, transformation induced plasticity (TRIP) steels of various categories and Martensitic steels. Four special considerations must be taken when pulling these materials.
One, heat may or may not be permitted based on the metallurgic category and the specific make, model and application of the steel. Since even low heat can destroy the properties of most of these steels, heating typically isn't recommended unless you are planning on replacing the part being heated. Even the welding process Gas Metal Arc Welding (GMAW) commonly known as MIG, isn't recommended.
Two, sectioning may not be an option. The entire part will require replacement at the factory seams identified by a manufacturer's procedure. This guideline is not limited to high-end European vehicles. Domestic manufacturers use the new steels in popular vehicles. The Dodge Caliber and Jeep Compass use boron alloyed steel in their inner B-pillar, roof rail and A-pillar and may never be sectioned.
Three, due to the strength of the steels reaching upward of 165,000 psi, pulling is extremely limited. These steels are more brittle and have a tendency to crack or incur micro cracking of their structure when bent. You need to inspect bend areas with a dye penetrant to ensure the structural integrity of the part.
Four, the force required to move these steels (which typically are used in full-length reinforcement of rocker panels and B-pillars) can tear loose the attachment points and cause collateral damages to the anchor points. To prevent this, you must cut access into these areas to allow the attached parts to be pulled. Even then, the increased pressure required to pull these areas will require careful inspection during the pulling process, plus additional anchoring points to avoid damaging the anchoring area.
Aluminum structures
Aluminum poses similar challenges. You may be restricted from straightening based on the type of aluminum part and whether it is stamped, extruded or cast. Always check the vehicle manufacturer's guidelines.
Once aluminum bends, it is content to stay in its new shape while remaining work-hardened. Increased pulling force is required to move it. As with the new steels, this increased force requires additional considerations when selecting anchoring points.
Aluminum has a tendency to crack first from collision damage and again during the straightening process. Dye penetrant is required to check for damage. In most cases, you must heat the aluminum if you plan to reuse the part you are pulling.
There are significant heat range differences between aluminum and steel. The temperature sometimes used to heat HSS (1,200°F for two minutes) will melt aluminum. Aluminum requires a heat range of 400 degrees F to 575 degrees F to bend without cracking during pulling. You must never exceed the recommended 575 degrees F. Unlike steel, aluminum does not have a defined cumulative time limit. Aluminum can be heated for approximately two minutes and then allowed to cool naturally and reheated again as much as needed.
To properly pull aluminum, you must pull with small corrections to limit the cracking. This requires a combination of heating within the heat range allowed, pulling a little, stress relieving, then letting the pressure off, allowing the aluminum to cool naturally, and repeating the process. For more information about Aluminum Structural Repair, the I-CAR SSA01 Course – Structural Straightening Aluminum, provides a solid approach.
Conclusion
Today's pulling equipment (like measuring systems) requires a commitment to keep them in top condition. Each system should be cleaned and all moving parts greased weekly. You should use shadow boards and check, then immediately repair all accessories.
You should receive training by the equipment manufacturer on a regular basis, both in-house and at training schools. Independent training programs can help ensure your staff is properly trained both on the equipment and on the most efficient repair processes.
Pulling technology and processes are not the same as they were just five years ago and probably will change notably in another five years. You need to stay informed.
