Questions about the Primo truck tensioning procedure?

Hi everybody,

I’m a little confused by the Set Truck Tension instructions from the Primo build guide.

It states that for “new prints” you can fasten the bolts and nuts until they both sit against the plastic of the 3D printed truck part.
However, if I do that there’s still some play, when I apply a lateral force and the truck sits on the rail. It wiggles ever so slightly, which I guess means that not all five bearings must be in contact with the rail.
Maybe my prints are too old for this to work? What’s considered a “new print”?

Furthermore, I find it rather hard to know - when fastening the bolts and nuts beyond them only sitting against the plastic - how much tension should be applied!
Is it generally better to have some tension that you for instance feel when sliding the truck off the rail. It’s a feeling of a little resistance when you slide the truck on and some letting-go when sliding if off. It grips the rail a little, if that makes sense. Or should the truck be looser?

I get that the trucks should slide slowly downwards under a 45° rail angle, however how slow?

I fastened the trucks yet again after one year because at certain points they didn’t touch the rails which resulted in stutter. Also, Ryan talks about a light touch, but I figured, at least for me, he means compared to a car. I really did a light touch with all the screws and had to tighten them a lot more over the year. You can also tighten them later on, so don’t worry about it too much.

Yeah, but if you haven’t worked on a car yet, I guess it’s hard to get the comparative reference.

I’ve found that you need to take the stepper motors off, as well as the idler-belt-contraption, otherwise there isn’t enough room to fit at least my tools.

As considering this is a DIY CNC machine, some trial-and-error will be necessary to get things where you need them to be. Most, if not all, of us have assembled and disassembled our machines at least once. This isn’t a low level project for beginners, so its assumed you have a bit of experience with tools and such.
As far as how much to tighten, if you don’t have experience with torque and tension, you can buy a torque wrench that is for inch-pounds if you want a gauge to read. Otherwise you’ll have to tighten a little, check to make sure all of the bearings on the trucks make good contact with the metal rods, and tighten more if they aren’t snug.

Ok. Compare it instead to anything you’ve ever screwed together (wood joint, Ikea furniture etc.) and then realize that it’s only a tiny fraction of THAT torque. (If you’ve never assembled Ikea furniture… I don’t know what to tell you.)

What (I interpret) that we’re looking for is the absolute minimum amount of torque to keep the plastic from opening, and/or bring the trucks into true.

I use an open-end wrench on the motor side. 1/2" (for 5/16" bolts) or 13mm (for 8mm) and you can use a socket on the other side. Use the wrench to hold the bolt head, and the socket to turn the nut. If you’ve assembled it so that the bolt head is close to the motor, with the low amount of torque you’re using (The nylock should be the majority of the resistance) you can even use needle-nosed pliers to hold it.

I’ve had to adjust my trucks a few times. Apparently, it’s now been a year and a day since I started my project, I just got the “anniversary” badge yesterday from the forum, which I joined the day I placed my first order for Primo parts.

1 Like

The screws are strong enough to completely tear through or break the plastic. Try to think about in terms of position instead of tension. On a car, you need to tighten the bolts enough so they apply enough tension to keep the car together. In this build, you are using the screws to keep the parts in the right position.

In many cases, the screws are way over spec’ed for the amount of tension they are providing. This is to reduce the part count, and complexity, and give the plastic more screw to hold onto. But the consequences is that gorilla hands can break stuff.

As for your question about tightening the trucks. Go ahead and tighten the screws to make it square. Ideally the bearings are all touching and it wants to be square. But a little bit if play is normal. When the machine is finished, there will be 4 motors holding it square. They will do more for your quality than the truck alignment.


You can do it with a lot of fiddling and swearing, trust me. :stuck_out_tongue:

1 Like

That’s why mine is square. :smiley:

1 Like

I don’t think that would help much, since there is no indication about how much tension should be applied for this part of the guide.

For me these comparisons are far to general. It’s apples and oranges.

I installed the bolts like shown in the guide. The nylock nut behind the motor sits fairly deep within its cavity in the 3D printed truck. I’ve tried with two different wrenches - open and closed - and neither could reach it. I’ll try pliers next time. For now I’ve unmounted the motors.

That’s what I specifically ask about above! I even give a description of a looser and tighter tensioned configuration that I’ve tried.

Squaring comes later. I’m following the guide and tensioning the trucks with a loose, uninstalled rail.
Should the bearings just barely touch the rail and pretty quickly slide down if you hold it at a 45° angle, which the concerned paragraph of the guide seems to imply, since it recommends to just sit the bolts and nylock nuts against the plastic?
Or should the truck slide “slowly”, when the rail is held at the 45° angle, like the guide proposes in the next paragraph? This would entail that the bolts and nuts get tightened far more.
I’ve tried both, I just want to know which one is the right one for now.

That’s good to know.

Thanks for all your replies, everybody!

For me it has been this, everything else produced chatter.

1 Like

I just tighten until they touch.

There, fixed it for you…