Expanding possibilities with spacers:

With all telescopic spreader bars one of the main issues is balancing strength of material with material selection that allows the two pieces to slide inside of each other nicely. Every person I have ever spoken with that has been involved in building telescopic spreader bars has built one that didn’t slide very well after paint. The following are my tactics for dealing with this issue.

Go With What Works:

Most of the spreader bars that I produce get powdercoated. There are a few reasons for this but what I can tell you, is powdercoating goes on very thick and the chances of clearances being different post paint is very real and needs to be accounted for. There is however, a tried and tested combination of tubing sizes that has worked every time for me. That is selecting an outside tube that is 0.5” larger in size than the inside tube and has a thickness of 3/16”. For instance 3×3 tubing slides inside of 3.5×3.5×3/16 really well. This combination leaves 1/16” of clearance all the way around the tube which is enough for one layer of paint (remember you don’t paint the inside of the one tube) and clearance for the weld seam.

Don’t know what a weld seam on tubing is? Check this materials article out Stocking Materials for Spreader Bars

If you can use a design that falls into this category that’s great! It is by far the most efficient combination to use. The limitations of this design are that despite having free control of the thickness of the inside tube you are stuck with a 3/16” outside tube which will have an inevitable limitation for regional buckling because of the thin wall.  Check this other article out for 5 tips for getting the most out of your 3/16 tube.

Put a spacer on the inside tube:

If you are in a situation where you can’t quite get the sizes to work out one option you could have is to repad the entire length of the inside tube. The times I have done this it is generally on larger sized bars and happens for one of two reasons. The first reason to need to do this is the load on the pin requires a thickness greater than what is available in that tubing size. For instance, I don’t believe 3x3x0.5” tubing is easy to get but if you need 0.5” thickness to give enough surface area for your pin then repading the tube is your only option. The other situation that I see this happen in is when you get into the larger sized bars and the tubing starts to skip sizes. Like you can get 6” tubing but not 6.5” so you are left trying to get a 6” tube to slide into a 7” tube. Repading the whole tube is by far the most expensive option available. If you think about welding a plate down each of the 4 sides of a tube that is 30ft long you have effectively gone from the 20” of weld required to attach the end connection to a maximum of 240’ of weld. Wow.

I often see engineers that sporadically get involved in spreader bars choose this option. I believe its half because it’s an easy solution from an engineering point of view but also because you get trapped as an engineer re-running your calculations because as you repad the tube technically you could use a smaller tube to handle the buckling. If you do find yourself using this tactic I have four tips to make it easier:

  • Use appropriately sized flatbar rather than plate sheet to make it easier.
  • Weld the repads down the length of the tube before drilling your holes. This will ensure proper hole alignment and dramatically reduce your headaches. Remember you are probably doing this because you need more bearing surface on your holes so if they are off by 1/16” you will only engage one surface. This will also save you from the worst case scenario where you are off by half a hole – in which case you should just start over.
  • Consider stitch welding the repad next to each hole and skipping the parts of the joint between the holes. If your hole centers are every 12” you could consider 4-6” of weld every 12” centered on the holes. But consult your engineer on a case by case basis.
  • Split the repad that lines up with the seam into two so that it has enough clearance.

Repad the Outside Tube:

This is a fairly common solution to the problem as well. The quick and dirty way to approach this is to weld metal spacers to the inside of the larger tube close to the opening and potentially matching repads at the base of the smaller tube. This will allow you to remove any slop between the tubes by adjusting the thickness of the repad being welded in. The high-tech solution which is common on crane outriggers and booms is to have a bolt in UHMW (Teflon) slider that can install. Either of these options are great and can potentially make a nice spreader bar. Just be sure you are thinking about assembly/disassembly before you start so you don’t weld yourself into a bar that can’t come apart for service.

Use Bearings:

This is my personally favorite option of getting two tubes to slide inside each other. There aren’t very many manufacturing companies out there using this strategy which is a shame because it makes an amazing product. The idea is to design your spreader bar with tubing that works best from an engineering point of view regardless of clearance. Ideally your outside tube will be 1” larger than your inside tube.  You then cut specific brackets and install roller bearings to maintain the perfect fit.  You put 2-4 bearings around the outside of the larger tube and 2-4 bearings on the butt plate of the smaller tube. You have to be careful to do this in a way that allows you to disassembly the two pieces because eventually the bearings will need maintenance. The finished product is amazing and even 80ton 60ft bars can be slid open or closed with one hand. If memory serves me correct I believe the bearings I use are around $20 so it isn’t a huge cost consider how well it works.

I hope that gets your creative juices flowing and you re-think your strategies for designing and building telescopic spreader bars. All of the Heavy Spreader Bars in the Online Store come with bearings standard but if you want them on other combinations of bars please let me know. If you have questions or comments please leave them below. I also wouldn’t mind hearing from everyone that has ever built a spreader bar and messed up the clearance between the telescoping components so I know I am not alone in this.

Thanks for reading this article and hopefully we are all on the same page with how to talk about spreader bars now. I will probably update this document every time a new term comes up but for now it is what it is. If you enjoyed the article and want to receive more update from us please take a moment to sign up for our mailing list. If you are just starting your journey into manufacturing spreader bars you should read this article next: An Introduction to Stocking Materials for Spreader Bars.