The All Steel Front End

[Jim Allen]

The first all steel front end was used by Krasnorutsky in 1977. This design was later used by the FMV team to win world championships in team race. More information about this history can be found on Google's web site under The FMV Story.

Bearings used in the all steel front end are of the hybrid type; silicon nitride-Si3N4(NBD-100 or SN-101C) balls & AISI 52100 steel races. All bearings (6000& 6002) used are SKF brand; ABEC 3 grade; with T9H polyamide retainers. The entire run (1000 pcs) of each size (6000 & 6002) of bearings was purchased to ensure all balls were of the same size & the special radial clearances were to specification. Radial clearances are as follows: (#6000-.0011 to .0015) & (#6002-.0014 to .0018). Ceramic balls of the correct size were purchased from Cerbec Saint-Gobain to replace the steel balls.

Special equipment was designed to check the radial clearance of all purchased bearings & an electronic micrometer was used to measure ball size. Hardened steel front ends when correctly made with hybrid bearings will run 800 to 1200 rpm's faster than standard aluminum front ends; including aluminum bar stock front ends; even when ceramic bearings are used. Hardened steel front ends maintain all of their performance gains 3 to 5 times longer than standard aluminum front ends. These front ends have been throughly dyno tested by myself & by Brian Charney in oval racing. They have been tested in .40, .60 & .90 size engines with excellent results.

Full hard (ground OD & ID; Rc-63 Rockwell hardness; O-1 tool steel) front ends are "shrink fitted" (.001 interference fit) into 2024-T351 crankcases which have been pre-heated to 350 deg f. Placed between each front end & crankcase assembly is one .003 to .020 machined brass shim gasket. This gasket provides perfect centering of the crankpin on the cylinder centerline for each assembly. Assembly of the heated crankcase & the steel front end are done in a fixture under pressure until the crankcase cools to room temperature. Four 8-32 screws tightened to 50 in. lbs. with # 271 Locktite completes the assembly.

The inner races of both front bearings are mechanically fastened to the crankshaft as per FMV method, allowing the axial play to be pre-set at .003 to .004. Since all parts made of steel have the same thermal coefficient of expansion; the axial & radial play remain constant during engine heat up.

I will post pictures of tools & fixtures used; explanations of bearing disassembly &

assembly; & critical numbers of bearing press fits.

 

[Jim Allen]

As promised a series of pictures have been posted showing a fixture used to check the radial play & roundness of a typical deep groove ball bearing. This is the standard type of fixture used for this purpose & one fixture can be used for many different size bearings by club menbers. No type of front end should be assembled before determining if the bearing being used meets the required specifications of radial play & roundness. Not doing this necessary evaluation can invite poor performance or possible early failure.

Future posts will talk about the hardened steel front end; giving critical fit numbers; methods used to build & fasten this unit to an aluminum crankcase.

Jim

 

[Tim_Duggan]

Jim,

Is it common to find a bearing out of spec? Also - have you found any particular size / brand of bearing as being particularly problematic?

I must confess - I do not test new bearings at all and replace them seasonally or if a problem occurrs in a shorter period.

Tim.

 

[Jim Allen]

Tim,

Economy bearings (Chinese as an example) can be a problem in any size. Henry Nelson purchases the entire run of SKF (Canada) bearings (1000 pcs; A3 tolerance; S clearance) of a particular size that are made to specs to ensure consistent ball size. These have metal balls which are replaced with ceramic balls for my use. Each bearing I use is checked for correct radial play & roundness with my fixture. All tested bearings have been good to date. In the past I had a problem with some GMN bearings that did not have the indicated radial clearance. When using bearings with insufficient radial play in any type of housing failure will be rapid because the necessary press fit reduces the radial play drastically. I have found, it is better for our bearings to be looser than tighter. Any off the shelf bearing from any bearing supply house should be checked very carefully! Let the buyer beware!

Jim

 

[Terry Keeley]

As always simply awsome stuff Jim, please keep it coming. I can spend hours poking around your Gallery looking, pondering, thinking. The quality of your digital photos is very impressive too!

I have a couple Q's about bearings if you don't mind?

When the radial clearance varies, which component(s) are out of round? Can the "defective" parts be found and replaced to make a good bearing?

I take the load directly on the front bearing in all my boats so I watch that bearing closely for wear which I measure as end play. To measure it I hold the engine in a vice, put an indicator on the front face of the flywheel and pull the crank in and out.

On some new good quality bearings the end play can be as low as 0.002" (cheaper bearings show up to 0.007" new), but when they reach about 0.005" I replace them. They're all supposed to have a C3 fit.

Is this end play in effect just the radial clearance you're measuring but "translated" in the axial direction?

ps: Here's a direct link to Jim's Gallery:

https://www.intlwaters.com/gallery/thumbnails.php?album=225

 

[Jim Allen]

Terry,

Out of roundness conditions can be found in the inner or outer race, or both. Some Sweedish tether car competitors have said they are re-grinding races to correct out of roundness conditions. I have not done this; to much work for model boat use. Doing this requires different ID front ends & different crankshaft OD's.

Yes, the amount of radial play effects the axial play. A problem will arise if a bearing which is susposed to have a certain amount of radial clearance has less or the interference fit in the housing is greater than designed. When these two conditions are present at the same time any bearing, any all metal or ceramic hybrid bearing will fail rapidly.

Jim

 

[Jim Allen]

In my opinion, heavy interference fits should be used on model engines operating at high rpm's. Also, the inner races should have a heavy push fit & be mechanically fastened to the crankshaft. To achieve this requires special (S radial clearance) bearings that cannot be purchased over the counter at typical bearing supply houses.

These high radial clearance bearings can also be used in properly sized aluminum front ends; but without the inner races being fastened to the crankshaft because of the rapid expansion of aluminum.

I have posted more photo's about front ends. Will try to do geared twin setup next.

 

[Tim_Duggan]

B)

Will try to do geared twin setup next.    

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Cool - that's something some of us on IW had been discussing at length a few years ago. B) B)