On Wed, 13 May 2015 15:13:06 -0700 (PDT), Sir Ridesalot
<
i_am_cyc...@yahoo.ca> wrote:
>Only those who have no knowledge of or who are silly will assemble
>a steel pedal spindle to an alloy crankarm without greasing
>the threads first. Unless of course they don't care if they end
>up with the exact problem the OP has. When steel corrodes it expands.
>That plus the corrosion caused by dissimilar metals can 'weld' steel
>to alloy. This is the same thing tthat often happens to ally
>stems inside a steel fork or an alloy seat post inside a steel frame.
>
>To tell people to further increase the problem of dissimilar metals
>by adding locktite is simply irresponsible advice.
>
>Cheers
My apologies if this is a bit deranged, but I'm recovering from an
afternoon at of drill and fill at the dentist and feel lousy.
Long ago, when I was designing marine radios, the issue of galvanic
corrosion arrived on my desk. We were using Anodyne plated aluminum
PEM nuts on an aluminum chassis to hold in PCB (printed circuit
boards) with stainless steel 4-40 screws. The dissimilar metals and
galvanic compatibility problem is somewhat similar to the steel pedal
and aluminum crank arm problem, with the added bonus of a tin/copper
PCB.
Everyone had an opinion on the matter. The issue was mostly over how
much money would be spent to prevent corrosion. 100% stainless would
be nice, but also expensive.
I had my own opinion, but didn't want to risk being wrong. So, I
built a test fixture out of roughly 1ft x 1ft square of 0.062 6061-T6
sheet metal, swaged in a variety of aluminum and stainless standoffs,
attached a copper plated PCB, and inserted a mix of aluminum and
stainless fasteners. Protective coatings were also suggested, so I
applied an assortment, which include grease, Loctite red, zinc
(galvanizing) flash plating, Anti-Seize, etc. I recall about 120
different combinations with some duplications. I also punched some
big holes in both the aluminum sheet and the PCB between the PEM nuts
to equalize the exposure.
I dragged the whole mess down to Viking Labs and had them run a
MIL-STD-810 salt fog test.
<
http://www.element.com/services-index/saltfogsaltspraytesting>
24 hrs is the equivalent of 10-15 years exposure, so I ran it for only
10 hrs (mostly because longer would have required paying the lab for
overtime).
The results were mixed. What survived best was aluminum PEM nuts and
flash galvanized (zinc) plated stainless screws.
<
http://www.finishing.com/160/63.shtml>
Without the plating, the aluminum PEM nut and stainless screws fell to
the bottom of the list. If the bicycle pedal manufacturers would
plate their steel threads with zinc, there would be less of a galvanic
corrosion problem.
I don't recall the exact order of the various potions I test. Teflon
paste and anti-seize (Never-Seize??) both worked very well, but were
impossibly messy to handle and apply. Grease was somewhat better than
no grease, but still managed to produce stuck fasteners. I tried
different greases and oils, varying from vegetable oil to axle grease.
The best was the then new calcium sulfonate based marine greases.
<
http://www.royalmfg.com/calciumsulfonate/>
The low viscosity oils barely worked but were easier to un-freeze than
those with no oil.
The problem with all this is that the manufacturers of pedals want the
pedal to lock to the crank arm. If it freezes in place, all the
better. If it gets loose, the danger is not that it falls off, but
rather that the motion of the steel threaded part will mechanically
wear down or deform the aluminum threads. Adding grease to the pedal
threads will cause it to slip. If you over-tighten the greased
threads in order to help keep it in place, the grease will allow you
to apply more torque, eventually peeling the threads out of the
aluminum crank arm. I have a bicycle that I bought used with exactly
that problem. It's much the same reason you don't want to use grease
on the crank arm to bottom bracket square axle taper.
Ever notice the finish on the pedal threads? Most are slightly rough,
although there are some that are polished. The rough surface is
intentional as it provides the friction necessary to keep the pedal
attached to the crank arm. Grease the threads, and the friction is
gone. However, the lack of a filler leaves gaps for electrolytes to
creep in and promote corrosion. Moral: You can win.
Well, actually you can win, but it costs money and adds a little
weight. Enlarge the hole and permanently insert a steel threaded
insert into the aluminum pedal:
<
http://www.parktool.com/blog/repair-help/pedal-bushing-kit>
However, I would like to see a more substantial insert, possibly with
set screw locks, or a lock nut. The set screw holes can also be used
to inject penetrating oil under pressure just in case it does get
stuck. The insert then becomes a permanent part of the crank arm. The
stock pedal just screws into the insert normally. Since there are no
dissimilar metals, there's no galvanic corrosion. There may still be
some surface corrosion, but that can be handled with some flash zinc
galvanizing.
Yawn, enough for tonite. I can barely focus on the screen and my
spelling errors have deteriorated to where the spelling chequer is
having problems. Sleep beckons...
--
Jeff Liebermann
je...@cruzio.com
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS
831-336-2558