Zinc-Coated Hardware Question (outdoor application) 7

[TireDesigner]

Hey everyone,

I have a conundrum regarding hardware for a wheel used in pivot (agricultural) irrigation systems. These wheels have several polymer segments that bolt to an inner galvnized steel wheel. The hardware that we had traditionally used was zinc-plated, but as you can probably guess, some of our end users complained about corrosion on the hardware. As far as I know, the hardware never rusted to the point where there was any structural failure, it was just cosmetic. But regardless, we got complaints so we decided to switch to galvanized hardware. Our hardware connections consist of a pair of washers, a bolt, and a locking (nylock) nut. We switched to galvanized washers and bolts, but had to stay with a zinc-plated nylock nut, because we can't find any type of galvanized lock nut. We figured that since the galvanized bolt and washer act as a sacrificial anode, we could get away with just zinc-plated locknuts. Unfortunately, the galvanized bolt & zinc nylock nut combination is proving very difficult to assemble, as the galvanized bolt often binds in the nut. We have to find a different solution, and SS is cost prohibitive for this application (there are a ton of connections on this thing).

My initial thought is that we could swap out the galvanized bolt for a zinc-plated bolt, and hopefully the galvanized washers acting as sacrificial anodes in conjunction with the (admittedly limited) zinc-coating protection on the bolt and nut would be enough to last for several years. What do you guys think? The only other option we can think of at the moment is to use a standard (non-locking) galvanized oversized nut on the galvanized bolt. However, we'd lose the locking ability of the nut and would have to add a lock washer. I'm apprehensive about using lock washers on this application just because these wheels are in near constant use over uneven ground, and I'm worried about them working loose. Also, this galvanized nut is square shaped (instead of hex), so it would be a little bit non-standard and would make field service with a ratchet harder (which is something we need to consider).

Any insight would be greatly appreciated.

Thanks,

Richard

 

[racookpe1978]

I would recommend stainless over galvanized. And either over a Cad-plated piece of soon-to-be-rusted ugly piece.

Use a standard ANSI/inches nut-and-bolt if you can - then advertise that! - since your customers will appreciate being able to make their own fixes using parts from Home Depot, Lowes, and Tractor Supply Company.

 

[CoryPad]

Any of the products referred to as lockwashers are useless, so you should not consider them.

If you use hot dipped galvanized (HDG) fasteners including the nut, you can consider using a double nut arrangement to provide resistance to loosening. Or, you can use thread adhesive, which would be cheaper.

If you decide to stay with zinc plating, you can consider changing to zinc-nickel alloy plating which has much improved corrosion resistance compared with plain zinc plating.

Lastly, you could consider using zinc-rich coatings (e.g. one of the products supplied by Magni, Doerken, NOF Metal Coatings, or Atotech) on all of the parts instead of zinc plating or HDG. The zinc-rich coatings have high corrosion resistance but with small coating thickness so it does't bind like HDG can.

 

[TireDesigner]

racookpe1978, thanks for the suggestion. Unfortunately, SS is going to be cost prohibitive in this application. We have a ton of connections on this thing, and we're up against some very tough competition. Using SS hardware would kill our margin on this thing (which is already small). Also, I'm not sure if SS would work with our galvanized wheel. I'm worried about galvanic corrosion of the galvanized wheel due to the SS hardware. We already use standard ANSI sized hardware.

CoryPad, I've always been skeptical of lock washers as well, although I'm not sure if I'd go so far as to say they're completely useless. I know there have been many heated debates (including several on eng-tips) about the effectiveness of various lock washers. Having said that, putting a lock washer on here would probably be more for show than anything else. We could probably get away with just the flat washers and torqueing down the nuts quite a bit and that would probably be fine (although I have no actual data to back this up). I'm not a fan of using a double-nut setup on products that are going out to end users, but thanks for the suggestion. The Loc-tite is a good idea. I'll take that into consideration.

I'll look into the zinc-nickel alloy plated hardware. I wasn't aware that existed. I'll look into the coatings, as well, but I think that will be a no-go, as applying this to all of the hardware connections will be very time-consuming on a product that already has more labor built into than we would like.

 

[FastenerDude]

Instead of a standard lockwasher, I would try a Nordlock washer, also called a "cam" washer. They prove to be superior to a standard lockwasher. To say that a lockwasher is completely useless is a bit of a generalization. In the correct applications, standard lockwashers prove to be more than adequate.
The reason your galvanized bolt is not assembling with your zinc plated nut is b/c of the thick galvanized coating. Any time a galvanized stud or bolt is used, an over-sized tapped galvanized nut should also be used. This allows for the additional thickness of the coating so that the bolt and nut assemble.

Greg
System 22, Inc.

 

[CoryPad]

Greg,

NASA RP1228 refers to lockwashers as useless. Here is a nice collection of thoughts from Eng-Tips:

thread725-370111

 

[TireDesigner]

Greg,

Thanks for the comment. I hadn't heard of Nord Lock washers before. I looked them up, and unfortunately I think they will be cost prohibitive for our application.

 

[TVP]

TireDesigner,

The zinc-rich coatings that CoryPad mentioned are applied in bulk to the various fasteners instead of zinc plating or galvanizing. They are quite cost competitive with high quality zinc or zinc-alloy plating. Here are some links for more information:

http://www.swdinc.com/DipSpinCoatings.html

http://curtismetal.com/Services.html

http://www.chemplateindustries.com/atotech.html

 

[TireDesigner]

TVP: thanks for the links and the clarification.

 

[dvd]

Agree stainless / galvanized may be problematic:

http://www.eaa1000.av.org/technicl/corrosion/galvanic.htm

I particularly like the "small car" analogy - keep a small cathode/anode ratio.

What is wrong with the loctite suggestion and all galvanized hardware - if it is not factory applied, farmers are more than smart enough to use loctite.

 

[TireDesigner]

Thanks for your input, everyone. I really appreciate it. I think we're going to go with all galvanized hardware (no lock washers). We're pretty sure that with the torque applied in our assembly process it should hold up in the field.