New Tailwheel Leaf Springs too thick

How to keep the Cessna 170 flying and airworthy.

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Re: New Tailwheel Leaf Springs too thick

Postby gahorn » Thu Feb 27, 2020 12:59 am

Thx. Meanwhile, anyone so desperate to modify anything... I suggest that before one modified springs by using a belt sander etc, consider only modifying the TOP spring to get the stack to fit, as that top spring doesn't serve much purpose as far as spring action is concerned.
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Re: New Tailwheel Leaf Springs too thick

Postby hilltop170 » Fri Feb 28, 2020 4:56 pm

To piggy-back on gahorn’s comment above, I agree, thinning the top spring to bring the stack height into spec to fit into the bracket (without removing the lower riveted shim), would be the preferred process. However you do it, either with a grinder or belt sander, material removal should be done in-line with the spring axis as opposed to across the spring, perpendicular to the spring axis, which would create stress risers.

Also, if a C-clamp has to be used to compress the stack in order to make it fit, when the stack is installed and the C-clamp is released, there might be enough force released to damage the mounting bracket or hardware.
Last edited by hilltop170 on Thu Mar 19, 2020 4:59 am, edited 1 time in total.
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Re: New Tailwheel Leaf Springs too thick

Postby Larry E » Sun Mar 01, 2020 5:25 pm

Gentlemen

Thank you for discussing this problem.
I first removed the riveted aluminum 0.063" shim and was able to force the new leaf springs into the available space. Then this thread provided a lot more info, and I decided to take the assembly off and re-install the shim. I made measurements similar to what Bruce has done and found my bracket to be dimensionally similar.

The total vertical dimension at the aft end of the fish-mouth bracket is 1.055" with the shim removed. With the shim installed the dimension is 0.990".

When I clamped the new springs together - in a 6 inch vice - it was impossible to compress the 4 of them to anywhere near 0.990". If one adds the individual thickness' of the new springs they total 1.025". I couldn't get close to that on the airplane as even a 6" C-clamp could barely force the pack to 1.055"! It was still very difficult to install the set with no shim installed.

IMG_1762.jpg
Need a 6 inch vice to get 1.032" total thickness


It is possible that the springs were made with the bends positioned where they should be if the thickness is 0.180" but if the material is 0.040" thicker, the leafs will not fit together - there will be space between them.

IMG_1766.jpg
The 4 leafs do not fit well together


So I cleaned up the top 2 (shortest) original leaf springs and was able to use a 3" C-clamp to get a total stack dimension of 0.980" (using 2 new and 2 original leafs) which fits in the space available.
IMG_1786.jpg
2 new and 2 old leafs - fit well together AND fit in the space available

I suspect life will get easier for us if Univair is able to change the thickness of the top 2 shorter leaf springs to around 0.180".

Once again, the wisdom of this group has been tremendously helpful.

Thank you!

Larry
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Re: New Tailwheel Leaf Springs too thick

Postby Bruce Fenstermacher » Sun Mar 01, 2020 9:46 pm

So Larry for the record, could you confirm the actual thickness of the two old springs you used. Here is what I gathered from your earlier post but you suggest .180 would be better?
Longest : 0.320" thick U0542106
Next : 0.265" U0542105
Next : .195? old spring
Shortest : .193? old spring

Interesting that you measure your opening at .990 and I measure my new one at .951. Think the difference, and in some examples it is obvious, is if the AN-3 holes for the top angle plate are drilled out (or wallowed) to make the opening just wide enough to fit the pack one has on hand. My plate and bracket still have factory paint in the holes. They are so tight the AN-3 bolts cut slight threads in the holes. There is NO play. In fact I'd be tempted to drill them out slightly in the normal course of installation.

On the other hand, here is Jim McIntosh's installation posted at another thread. Look at the yellow and green arrows to see his top angle does not line up flush with the bracket. In fact it is angled to favor a wider aft opening. I was even wondering if those are still AN-3 bolts holding the angle bracket on or AN-4? Looking at the magenta arrows you can see he gets nearly the width of the top spring between the bracket and the end of the top angle bracket. Look at my new bracket below and see there is barely 1/64" slot.
Jim Mc bracket.jpeg

IMG_0626D.jpeg


I do think that you make a good point with the bends being a bit off cause of the difference in thickness from old to new.
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Re: New Tailwheel Leaf Springs too thick

Postby c170b53 » Mon Mar 02, 2020 1:34 am

Good eye Bruce, I had to replace my tail wheel spring retaining channel as it lost its tail (cracked) and the forward bolt hole was damaged by a previous owner who had omitted the shim. I positioned the spring pack and picked up the holes as the new channel comes new not drilled. The bolts I used are NAS equivalents to AN3’s . I also used smaller profile nuts which made the work easier reasoning the bolts are in shear.
I think I have the original top 3 springs, I’ll measure them next time I’m out at the hangar.
Larry, thanks for putting the bottom shim back in, a wise decision on your part in my opinion.
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Re: New Tailwheel Leaf Springs too thick

Postby n2582d » Mon Mar 02, 2020 5:44 am

For reference my bracket measures 1.009" between the shim and the tailwheel spring retainer channel. The top edge of the channel is about flush with the top of the tailwheel bracket.
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Re: New Tailwheel Leaf Springs too thick

Postby c170b53 » Mon Mar 02, 2020 5:53 am

I’d have to look to be exact but I would say I did that work somewhere around 2002 and replaced the main spring then and again somewhere around 2014-2015.
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Re: New Tailwheel Leaf Springs too thick

Postby Bruce Fenstermacher » Mon Mar 02, 2020 12:40 pm

Jim, I'm not following at all how the removal of a .063 shim at the bottom of the aft end of a spring pack can effect any bolt hole. I have no idea what bolt hole you refer to as the forward bolt hole. Could you be more specific.
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Re: New Tailwheel Leaf Springs too thick

Postby c170b53 » Mon Mar 02, 2020 4:29 pm

Yes I can see how that post could be confusing when not referring to the IPC. The shim I referred to is item # 10 figure 29 from the “B” IPC. I think I mentioned the channel could be replaced as a (my) solution to the thickening of the springs. I also mentioned using clay putty to measure gaps. It’s an extremely cheap, easy and often used method in aviation when you want to measure hidden dimensions. The previous owner had omitted the shim and failed to realize the result (crush) of cranking down on item #2, fig 29 AN6-23A bolt. As there was a gap, the channel aluminium gave way to the steel bolt hence the channel bolt hole bore was toast.Safe to say the channel, bolts and spring pack are subject to a fair amount of stress.
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Re: New Tailwheel Leaf Springs too thick

Postby Larry E » Tue Mar 03, 2020 2:11 pm

Bruce

In order to use the top 2 original springs I had to clean them up quite a bit.This meant taking off about 5 thou on each one. They now measure 0.190" and 0.188".

My suggestion for Univair to make them at 0.180" was based on the fact they told me the spec for the longest leaf spring was 0.312" and we have pretty well all seen that when we get a new one it's 0.320". In other words, about 8 thou too thick. Applying the same logic to the thin ones - if they plan to make them at 0.180" maybe they will arrive at 0.190".

There have been forum reports with members having leafs as thin as 0.170". And some have found their second longest leaf to be 0.235" - not 0.265" as most have recently seemed to be. There could be many reasons for this - including previous owners having to 'thin down' or salvage parts from other types in order to fit.

All of the above leads to me think we need to have a set of springs that can fit into an opening whose range is about 0.951" (your NEW, untouched bracket), to 1.000" (which might be more representative of what's actually in our well-used planes). As has been mentioned, there is a bit of 'slop' available by 'egging' out the 3/16" holes for the AN-3 bolts.

There is also the problem of bend axis location AND the amount of bend in each leaf. Both of these factors affect how big an opening will be needed. It sure would be nice to have the OEM specs.

Who'd thunk a few simple leaf springs could be so complicated!

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Re: New Tailwheel Leaf Springs too thick

Postby gahorn » Wed Mar 04, 2020 3:21 am

Larry E wrote:Bruce

In order to use the top 2 original springs I had to clean them up quite a bit.This meant taking off about 5 thou on each one. They now measure 0.190" and 0.188".

My suggestion for Univair to make them at 0.180" was based on the fact they told me the spec for the longest leaf spring was 0.312" and we have pretty well all seen that when we get a new one it's 0.320". In other words, about 8 thou too thick. Applying the same logic to the thin ones - if they plan to make them at 0.180" maybe they will arrive at 0.190".

There have been forum reports with members having leafs as thin as 0.170". And some have found their second longest leaf to be 0.235" - not 0.265" as most have recently seemed to be. There could be many reasons for this - including previous owners having to 'thin down' or salvage parts from other types in order to fit.

All of the above leads to me think we need to have a set of springs that can fit into an opening whose range is about 0.951" (your NEW, untouched bracket), to 1.000" (which might be more representative of what's actually in our well-used planes). As has been mentioned, there is a bit of 'slop' available by 'egging' out the 3/16" holes for the AN-3 bolts.

There is also the problem of bend axis location AND the amount of bend in each leaf. Both of these factors affect how big an opening will be needed. It sure would be nice to have the OEM specs.

Who'd thunk a few simple leaf springs could be so complicated!

Larry


I consider it defective logic to being assuming that we need new manufacture to be based upon presumptive alterations by unknown 170 owners.

The whole concept of FAA-PMA is that OEM parts are reverse-engineered such that newly mfr’d parts are exact replacements.

The problem (as I see it) is that we have 170 owners using parts intended for a different model aircraft (L19) but expecting bolt-on fitment as if one goes to buy an automotive replacment water-pump at AutoZone for your 67 Mustang. There is very little proof that the tailwheel bracket Bruce has is the bracket for which the Univair springs are intended to fit. Nor is it proven that the Univair FAA-PMA replacement springs will fit the brackets sold by Air Repair (whose sources at present are unknown. I.E, are they genuine Cessna? Or are they Ector mfr’d parts? Or contracted replacements? Or Military-contracted? Or ????

Airplanes, although mass-produced, are commonly fabricated and fitted by hand on an individual basis. If you don’t believe that, then just try to fit a genuine NOS Cessna 170/172 door onto an airframe. There is considerable “hand fitment” involved. I suspect we are discovering the same is true of this situation where we are fitting Univair springs to Air Repair surplus brackets of unknown manufacture.
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Re: New Tailwheel Leaf Springs too thick

Postby Bruce Fenstermacher » Wed Mar 04, 2020 11:31 am

The "kit" one can buy from Air Repair consists of individual parts collected. Every part has inspection stamps typical of the 50 and 60s time frame. The box they arrive in is the original box the bracket came in when it was originally packaged. In my case the box and bracket have stamps or labels indicating the bracket was made in June '62. All of the springs are painted with part numbers. 3 are Air Force gray and one is Army green. They all came in the original wrapper sealed with original old tape. I hated to cut it to look at the springs. One spring, the oldest was manufactured by Cessna in 1952. It is all on the original Cessna label. This is the green spring and it would be the shortest top spring and it measures .196" thick.

IMG_0628D.jpeg


The box and bracket where sent to Air Repair by Nell-Joy Industries of NY. I can not tell if they are the people who assembled the kit but with what I have no doubt are original Cessna parts made by either Cessna or their contractors. Jim Dyer seemed well aware of this cashe of parts and indicated there are plenty of them.

That spring I have made in '52 would be a very early L-19 spring by part number. likely representative of that found in a '52 170. The bracket I have has the most restrictive opening reported so far. I have no problem saying it represents what came on a new '53 170.
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Re: New Tailwheel Leaf Springs too thick

Postby c170b53 » Wed Mar 04, 2020 1:17 pm

There’s history, very cool and nice snag. One part of me says find spares, the other wants to keep my fingers crossed,
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Re: New Tailwheel Leaf Springs too thick

Postby gahorn » Wed Mar 04, 2020 4:10 pm

c170b53 wrote:....One part of me says find spares, the other wants to keep my fingers crossed,


My airplane was born 4 years after I was ...and is in better physical condition than I am...and will doubtless remain in better condition beyond me...unless one of us wrecks the other. And there are plenty of people better equipped to store the bazillion available parts spares than I am....and the next owner of my airplane will probably be able to buy them. I have the spare parts I anticipate needing in the near future (oil and filters) on-hand.

My Cessna original tailwheel bracket is inspected and airworthy and my main leafspring was replaced 300 hours ago with a Univair FAA-PMA that fit it perfectly. Other than another Univair main leafspring the only “spare” I own is an NOS Instrument panel cover and NOS gascolator (because of the glass bowl which I prefer despite FAA fears.)

IF one were to need to replace the bracket with an L19 part, then I would suggest one also acquire/use the associated L19 springs.

HIstory has shown that the L19 main spring is slightly thicker than the OEM 170 main spring. An L19 mainspring will likely be a tight/impossible fit into an OEM 170 setup.

If one’s airplane has it’s original 170 bracket I would suspect that the Univair main spring would fit (at least it did mine.).

There is no need to replace ANY but the main/lowest spring, IMO. The upper springs do not have any history of failure to my knowlege. Only the lowest (Main) tailwheel leafspring appears to have that tendency. The collective opinions of the Assoc’n indicate a 500 hour/cycle lifespan for that spring.

Polishing the lower trailing-edge of the spring just above the main spring seems a good idea to reduce the possibility of it working a “stress riser” groove into the main spring (which is where the majority of those I’ve seen fail have broken.)

YMMV
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Re: New Tailwheel Leaf Springs too thick

Postby c170b53 » Wed Mar 18, 2020 10:18 pm

Had a desire to measure the amount of material of the spring retaining channel protruding above the fishmouth fitting. ( the yellow arrow) in above pic). Gap measured .050 at the end ,( maximum dimension) of the fish mouth . Probably just bad workmanship on my part but it’s lasted in service at least 15 years and hoping for a few more :D
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