Mast Step Fasteners

[hilbert]

I picked up these light weight 316 SS mast steps in good used condition.
An online web site that sells these steps, also offers monel rivets at $0.90 a pop. That would add up quickly.
Is there any reason not to drill, tap and screw them in with 316 machine screws?


http://www.swi-tec.us/index.php?route=p ... duct_id=19

[jplloyd]

Rivets are definitely a better fastener for thin wall aluminum. Many aluminum alloys do not hold a thread well, and there's only going to be a couple of threads to grip. It's very easy to strip or cross-thread when tapping or screwing into something as thin as a mast. It's likely the specific alloy for the mast is chosen for extrudability/weldability, not thread holding. Rivets, on the other hand, are extremely robust, and easy to drill out and replace if they ever loosen up (not really possible with tapped holes). If you are planning to put weight in the steps, go with rivets. Much safer. You can probably get rivets in bulk a little cheaper.

[Steve Laume]

With 6 fasteners per step I don't think you have too much to worry about.

If you want to go to all the trouble of drilling and tapping all those holes, machine screws would work well in my opinion. Our mast sections are thicker than you might imagine. You do want to use fine threaded fasteners and tef gel. These steps are only going to see shear loads so there is not much strain put on the threads in this situation.

I would use rivets just because of the sheer number of holes you are dealing with and the fact that they are easier to align. Unless you are a really big guy, I would think aluminum rivets would have plenty of strength. My radar bracket is held on with a few aluminum rivets and that thing stays up there in conditions that I hope you would never be climbing your mast.

If you want to stay with stainless I would look for a bulk supplier and I can't imagine they would be much different in price than machine screws, Steve.

[hilbert]

Thanks for the replies.

I was considering machine screws not only because of the cost of monel rivets, but also the cost of a good rivet gun.
For me, mast steps are a safety issue so I'll use rivets. As jplloyd wrote, it will also be easier to repair a loose fastener.

Do others agree that aluminum rivets would be adequate or should I pop for monel?

[jplloyd]

Loads on the lower fasteners are almost entirely shear, but the upper fasteners not. Eyeballing the angle as about 45 degrees, the tension on the fastener could be as much as ~0.7 of the load on the step, which could be 2x the weight of a person climbing vigorously (or scrambling to keep balance). In this kind of configuration one of the fasteners is bound to be looser than the other, so the load will not be evenly balanced between the two. 500 lbs in tension and shear wouldn't be an unreasonable load specification, and 2-3 times that wouldn't be an unreasonable safety margin.

McMaster-Carr has 3/16 rivets in Aluminum rated at 310/500 lbs (shear/tensile) for about $15/250, 18-8 stainless rated at 950/1200 lbs for about $10/50; Monel rated at 740/840 lbs for $6.81/10 and 316 SS rated at 1320/1610 for ~$13/10.

I would go with 18/8 unless you're in salt water (or you're burly and climb your mast with vigor), in which case it looks like 316 stainless.

Based on the above I would definitely rule out aluminum rivets unless you can use a pretty big rivet (hard to judge exactly from the picture what you can fit).

[Russell]

Rivets are definitely a better fastener for thin wall aluminum. Many aluminum alloys do not hold a thread well, and there's only going to be a couple of threads to grip. It's very easy to strip or cross-thread when tapping or screwing into something as thin as a mast. It's likely the specific alloy for the mast is chosen for extrudability/weldability, not thread holding. Rivets, on the other hand, are extremely robust, and easy to drill out and replace if they ever loosen up (not really possible with tapped holes). If you are planning to put weight in the steps, go with rivets. Much safer. You can probably get rivets in bulk a little cheaper.

While you make a good point, keep in mind that goosnecks and boomvangs are typically attached to the mast with machine screws and the loads taken there are considerably more then the weight of even the biggest person stepping on a mast step.

That said, I agree with opting for rivets as others pointed out, for ease of install. Though I would go for SS rivets, not monel or aluminum. The downside there though is a SS rivet isnt going to pop with a basic rivet gun, you need a heavy duty one. But if you really want to tap all those holes, I wouldnt hesitate to climb a mast with steps installed with machine screws.

Dont forget to use tef gel if you use SS screws or you are setting yourself (or the next owner) up for a nightmare if they ever need to be removed. Rivets less so since they need to be drilled out anyway.

[Steve Laume]

This is also a really great excuse to buy a decent rivet tool as well. You will never regret having it on board along with a small assortment of rivets. With that and a battery powered drill you can fix or install a lot of stuff, Steve.