Simultaneous weight reduction and cost reduction is one of the most important development targets in the transportation industry.
That doesn’t just apply to traditional cars, trucks, and trains. It also applies to trailers, buses, and alternative transportation like e-cars and e-trucks.
These applications are deeply ingrained in the world of roll forming. Not surprisingly they have design best practices that are both similar and unique compared with traditional vehicles.
The best lightweight materials for cars might be different from the ideal materials for trailer components. Traditional automotive weight reduction ideas might not necessarily agree with e-truck weight reduction needs.
This roll forming design guide is dedicated to lightweight trailer and transportation components -- ignore at your own risk!
A 7-Step Roll Forming Design Guide
If your application sounds like what we’ve described above, your roll forming design challenges probably fit into these buckets:
- Material substitutions -- strength vs. weight
- Durability and hardness
- Formability issues, including cutoff and springback considerations
- Additional features
Aluminum is the favorite for many kinds of applications that require lightweight materials.
Steel, of course, is much stronger than aluminum. Conversely, aluminum is much lighter than steel. Talking in generalities (and not specialty or unique grades), you only need a little more aluminum to fabricate a shape with comparable strength to the same shape made from steel. Consider that:
- A cubic foot of standard-grade aluminum weighs 169 lbs.
- A cubic foot of standard-grade steel weighs 490 lbs.
There’s a second balancing act engineers are facing when choosing materials: cost vs. corrosion resistance. Aluminum costs 3.5x as much as steel, but unlike steel it creates a self-protective layer when exposed to oxygen.
In the case of truck frames and trailers (and possibly other applications), there are several popular aluminum grades:
Note that 6061 and 6063 are “extrusion-grade” types of aluminum, which are difficult to find in coil form. For this reason, roll forming suppliers generally avoid these grades. “Roll forming-grade” aluminum types include 5052 and some 3000 series grades.
But all lightweight component designers already know about aluminum. Let’s talk about something more interesting -- special steels.
Engineers have used specialty steels instead of conventional steel for years. These range from HSLA (high-strength low-alloy) steels to UHSS (ultra-high-strength steel) material.
The latter can reach tensile strengths up to 232,000 pounds per square inch (PSI). These steels already contribute to weight reduction efforts in traditional automobiles.
Your application, however, may not require such a powerful material. That’s why forward-thinking roll formers are recommending a certain HSLA steel for more and more projects.
80,000 PSI vs. 30,000 PSI Steel in Action
The alloying used to make steel has come a long way. The auto industry has started using much higher-strength steel in chassis components because they can make thinner sections with equal strength.
Now, why would we push for the use of HSLA steel in a project like yours? The ability to maintain structural integrity while decreasing the amount of material you need.
The video demonstrates the difference between two steels with differing yield strengths -- one at 30,000 PSI (30 KSI) and one at 80,000 PSI (80 KSI). As you’ll recall, yield strength is resistance to bending and permanently deforming under stress.
Standard, commercial-grade steel has a PSI of 30,000. It’s pretty low-cost, which explains its popularity. Here’s the problem: It still doesn’t match the cost-effectiveness of 80,000 PSI steel..
You’ll see in the video that there’s some fight in the 30,000 PSI steel … another reason why it’s popular for many fabricated assemblies. But behold the 80,000 PSI steel, which has about the same price per pound. It’s much tougher to bend the shape.
In other words, you might be missing out on a cost-saving opportunity by saying “But that’s what we’ve always done.” Remember that raw material can make up 60-70% of the overall price of these parts! Using a little less material goes a long way.
Note that while some metals and plastics have better strength-to-weight ratios than some UHSS and HSLA counterparts, the weight reduction achieved with these materials is almost always offset by higher cost.
Other Types of ‘Durability’: Hardness
The definition of “durability” depends on who you ask -- we mentioned corrosion resistance already (did we?), but there’s more to the story. On top of that, too much so-called “durability” may be a bad thing.
For the sake of this article, we’ll narrow in on two qualities under the umbrella of durability: strength and hardness. These two help determine the impact of collisions on your trailer, bus, or e-vehicle. (There are more intelligent people than us figuring out impact collision physics, so we won’t pretend to be the leading experts here.)
What we can tell you is that, weight aside, determining properties for trailers and other transportation components often comes down to balancing strength and hardness.
Strength could mean a part is strong enough to be a little flexible so it can withstand constant stress. But it can also mean the product is like an aging NBA player -- so stiff it doesn’t “rebound” at all.
Your roll forming supplier should assess the desired metal’s properties and match it up against the complexity of your part’s bends.
An example: An e-bicycle needs a certain steel alloy that’s flexible enough to absorb shock, but is durable enough to serve as a sturdy frame. A grade of roll formed steel that’s too rigid would make the ride less smooth, and the material might even crack if the cyclist hits a bump.
Hardness is the ability of a material to, without becoming brittle, resist:
- Surface indentation
When designing structural metal components for a train, durability usually refers to surface hardness. The metal is case hardened, heat treated, and quenched to make the outside ultra-hard. The inside remains softer.
In a vehicle chassis, however, the engineer must design door crossmembers that maintain their shape under normal driving, but absorb energy during a crash … but not so much that the impact force spreads to the rest of the vehicle.
Do either of these lessons apply to your own application? If so, don’t forget them.
Forming Advanced Materials
Forming higher-strength steels can be a challenge for your roll forming vendor. As long as your partner has the capability to accomplish it, your result may be worth the trouble.
In recent years, roll forming has swiped many traditionally stamped jobs in the automotive industry. Your vendor’s (and your design engineer’s) biggest struggles in using high-strength steels will usually be poor flatness and see-sawing mechanical properties.
One way your manufacturer can address this is controlled bending, which guides the material by rollers from both sides along the entire forming line. We’ve seen this produce panel components from martensitic alloy steel with consistent tolerances of 0.02”.
Along with operator experience and skill, it’s critical that the cut-off press following the forming machine has the tonnage (force) required to cut each component off at the end of the line.
This means you’ll need to know the range of the material’s yield strength. A strip of stainless steel might require 2x the force as the same length of mild steel. our roll forming company will make you aware if they don’t have a large enough cut-off press or if they decide to pre-punch part of the cut-off prior to forming allowing a smaller cut-off press to shear the rest.
The Springback Formula
Because you need high-strength materials for trailers, buses, and e-vehicles, you’ll have to deal with springback compensation. Materials with higher yield strengths and more springback (stainless steel, structural steels, HSLA steels, etc.) require more work to be formed.
Let’s back up and define springback. Springback is the general distortion of a part after it’s formed. It’s named so because literally your component springs back toward its old shape.
But hey! Since this is a fairly predictable outcome, you can work around it. The main predictors of springback are the metal’s yield point (the point at which a metal will stop reverting to its original shape) and elastic modulus (the change in stress with an applied strain).
Here’s a quick bending springback calculator of sorts to get you in the right frame of mind for degrees of “overbend” required. These numbers assume there’s a 1:1 relationship between the metal’s thickness and the part’s inside radius:
- Cold-rolled steel: 0.75 to 1.0°
- Hot-rolled steel: 0.5 to 1.0°
- Mild aluminum: 1.5 to 2°
- 304 stainless steel: 2 to 3°
- Copper: 0.00 to 0.5°
- Brass: 0.00 to 0.5°
Knowing how to make wise springback predictions will help you make better roll form tooling selections, especially for bends with some serious radii.
The size of a component also plays a role in determining whether it can be run on any particular roll form line. This will vary by contractor, so don’t assume your component is one-size-fits-all.
Prior to forming, the strip width of the metal required to produce your profile is a significant factor in design-for-manufacturability on a roll forming line. If the strip width is less than the roll space of the shafts, the profile is probably formable for width.
Also, the profile’s final height must be less than the vertical distance between the top and bottom shafts including space for the forming tools themselves. Otherwise, the mill can’t run your profile. As the profile shape gets taller, your supplier will need more tool steel to accommodate the height. In general, the larger the overall area of the profile is, the higher the tooling cost is due to more tool steel required.
How Complex Is the Profile?
More non-90 degree bends and larger radii dictate more creative applied forces and more tooling design time. You know those mazes that restaurants put on their paper placemats to keep kids entertained until the food arrives? If your design looks like that, you’re going to pay for it -- literally.
The more bends a profile has, and the more dramatic those bend angles are, the more forming stations the part will need.
We’ll use extreme examples to drive the point home: Think of an angle profile with short legs (a single, 90° bend angle) requiring only a few passes of tooling. Now for the extreme opposite: A complex profile with 12 or more bends, including hem (180°) bends, would require 20+ passes of tooling and forming stations.
The number of bends in your model determines your roll forming tooling design -- and the overall project cost. However, once these tools are designed and tested to complete the complex profile, the forming time is equal to the single bend angle. So in essence, as the parts are formed you get 12+ bends for the same cost a single bend!
Never forget one of the biggest advantages of roll forming: in-line features!
In many cases, components of industrial assemblies have pierced or embossed areas. Most roll forming companies have equipment that includes precision strip feeding and pre-punching to add these types of features to the material either before or after it is formed.
Really, “roll forming” has meant more than just roll forming for decades. Many shops offer in-line auxiliary processes and features such as:
- Identification marking
- Tabs, longitudinal bends, edge conditioning.
The capabilities and efficiency of roll forming lines allows for a single, continuous process of interconnected activities starting with a coil of metal on one end and a shipping container at the other end.. Keep this, and the ability to add in-line features free of charge, top of mind when considering your lightweight design components. You can get away with more than maybe you’d expect without breaking the bank.
Are Aesthetics Important?
Are your parts decorative? Have you even considered roll forming aesthetic components as an option?
These seem like no-brainers, but in the heat of battle we sometimes overlook the obvious.
If your part will be public-facing and needs to look good, tell your roll former. The engineers will be able adjust accordingly, and are perfectly capable of giving you a final component free of tooling marks. The smooth polish and gentile radiuses of roll forming tools are especially suited for cosmetic surfaces. For additional protection, consider a peel-off PVC coating on your raw material.
Conversely, if your part will be inside a larger product and will never see the light of day, consider ditching aesthetic touch-ups that drive price upward.
If you haven’t been considering aesthetics at all, maybe it’s time to start. One increasingly popular use for roll forming is the addition of “retro” decorative elements on the outside of trailers, campers, and other vehicles.
Freedom at Last
Today, many high-strength parts are manufactured economically on roll forming lines:
- Door beams
- Crash tubes
A material’s strength is not a challenge for a roll form machine as long as the tooling design, press punching, and forming steps incorporate the proper forces. As a bonus, roll forming tools can produce millions of parts with minimal wear which maximizes your investment.
To sum it up: Roll forming can be done very successfully if you follow the rules above. Your engineer’s and the vendor’s engineers should collaborate on a custom solution that allows you to use high-strength steel while reducing your part’s wall thickness. Add inline punching for further process savings. The freedom to design parts of high strength, and lower cost will become evident once the fundamentals of roll forming are applied.
How far you take your idea all depends on the creativity of the collaborative design team Dahlstrom can help you set up... For more advice on roll forming design considerations, here’s a free e-book below!
Topics: OEM Roll Forming
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