| Author |
 Topic  |
|
|
Brian Kelly
USA
526 Posts |
 Posted - 14/01/2009 : 17:57:11
|
Allow me to raise a question that I asked previously on the forum with regard to measurements of oil grooves in the main bearings of the crank, to which there was only one response.
In Bob Jones' article on engine rebuild, "MG Road Cars. Vol.1" page 96, he states that each main bearing white metal should have a generous oil groove at least 1/4" wide and 0.150" deep.
The stated depth of the oil groove 0.150" surely cannot be correct. As the original steel bare sleeve on the front is constructed with a side wall of 0.125" and the rear bearing sleeve wall being 0.153", bare of white metal.
Consider the front sleeve, for instance; 1.933" O.D. Original crank journal 1-5/8" (1.625") With an oil groove of 0.150" deep on each side gives an overall 0.300".
Am I missing some thing Here?
My question being, what dimensions have others used for the oil grooves?
Thoughts please.
Brian. |
|
|
Bruce Sutherland
United Kingdom
1564 Posts |
Posted - 14/01/2009 : 20:07:37
|
Hi Brian,
Not being an expert in tribology/lubrication (but others in the forum may be?), but what I recall is that in a hydrodynamic bearing (which big ends and mains are) that the oil pressure within the bearing to support the bearing load is developed by the rotation of the journal within the white metal or shell bearing..........i.e. an adequate supply of oil, NOT the pressure in the supply-line is all that is required. The greater the clearance in the bearing will tend to reduce the hydrodynamically-generated pressure.
If my logic is correct, excessive size of oil grooves will tend towards greater bearing clearance hence reduced load capacity; therefore the oil groove size just needs to be big enough to get the oil there, the rotational hydrodynamic effects do the rest.
The groove dimensions you quote seem quite large and I suggest that looking at a similar diameter modern shell bearing and its oil grooves might give you an answer.
Looking forward to an expert to correct my errors!
Bruce. (PB0564) |
 |
|
|
g waiting
United Kingdom
727 Posts |
Posted - 15/01/2009 : 18:08:24
|
Brian,
I usually make them 0.040" deep .
Garry |
|
|
|
Brian Kelly
USA
526 Posts |
Posted - 19/01/2009 : 02:13:53
|
Thanks for the input, gentlemen,
The figure quoted as 0.150" depth was obviously a bogus / typo. It would be physically impossible to have a groove that deep in the white metaled steel tube scantling and then press it in to it's housing. It would collapse under pressure as it would only be a few thou thick at the wall of the groove.
Before my engine problems I had oil grooves to a depth of 0.035" but I doubt that this measurement was due to the bearing failure.
I tried to contact Bob Jones by e-mail for some form of confirmation but to no avail. I can understand him being too busy.
Back to the drawing board. Thanks guys.
Brian. |
|
|
|
Bernard
United Kingdom
69 Posts |
Posted - 06/02/2009 : 18:36:12
|
Hi Brian,
Should have replied sooner but here's my six cents worth. Yes I noted the 150 thou looked a bit ambitious particlarly when using a sleeve for the larger crank and it should probably be 50 thou. I used 35 thou groove with a slight shamfer and widened the groove to match the crank feed hole supply. Provided they line up I cannot see any reason for it to be wider. I understand this grove on the J not only supplies the rear bearing but the rest of the crank and big ends so flow is important. At Beulieu I discussed a crank which had been modified with a rounded groove positioned opposite the bearing sleeve groove which effectively increased the flow capacity. However I was not going to machine my new crank. The sleeve also had been modified with an additional small hole 1/8'' further to the rear which linked to the block oil supply via a small groove in the rear of the sleeve. I liked this idea as it helped with oil supply further to the rear of the bearing. However as my sleeve was already in place I did not pursue it.
Happy engine building.
Bernard |
|
|
|
Bernard
United Kingdom
69 Posts |
Posted - 06/02/2009 : 18:46:37
|
Brian,
Apologies, the 35 thou stated should be 40 thou. This was the depth of the groove in my original bearing sleeve and the engine had run with this for some time-but not raced.
Bernard |
|
|
|
Dolts
United Kingdom
1129 Posts |
Posted - 10/02/2009 : 14:57:05
|
Hello Brian & Bruce,
I have the advatage of working all my life in a technical sales role for Vandervell so pleased to help with any engine bearing queries.
Firstly I have great respect for Bob Jones' technical ability, so his advice is always good.
Bruce, your advice is spot on, oil groove size is not critical so long as its enough to get the oil flow to the conrod bearings in fact most modern engines only have the block half grooved.the critical thing is oil film pressure which is very dependant on bearing length ( think of a cylinder,its not width !)
If you put an oil groove in a bearing it behaves like two bearings with half the length of a plain ungrooved bearing,This greatly reduces the oil film thickness, one reason why so many main bearing designs seemed so long--- you should see my 1925 Singer.
Time I republished my "Bearing Design" paper altered to suit MMM engines
So -- the simple answer is don't worry ,I have never bothered to measure an oil groove in 40 yrs of playing with engines.
rgds Tony Dolton |
|
|
|
Bernard
United Kingdom
69 Posts |
Posted - 10/02/2009 : 17:12:52
|
Hi all,
Fascinating stuff, look forward to the bearing design paper Tony. So with regard to the groove size for the MMM engine how big is big enough?
Bernard |
|
|
|
McEvoy
United Kingdom
252 Posts |
Posted - 10/02/2009 : 18:31:03
|
Another interesting topic and it may be of interest that back in the 50's when having a new crank made for one of the R s we tackled the oil groove & hence flow problem in another way on the advice of Robin Jackson. At that time thin wall bearings were not readily available so we increased the journal size as much as the con-rod would allow still using a white metal bearing material but instead of just having an oil hole in the crank pin we had a slot feeding out on the "low pressure" side of the pin.
Another way of increasing flow albeit a bit more expensive than altering the bearing groove.
Certainly like to see Tony's thesis - never too old to learn !
Bob |
|
|
|
Brian Kelly
USA
526 Posts |
Posted - 11/02/2009 : 04:18:41
|
Thank you gentlemen for the advise and comments.
Bernard mentioned the modification for an extra groove fed by a 1/8" hole in the rear bearing sleeve. I presume you were referring to a J type or another 1-1/2" dia. crank (?), as the larger 1-5/8"crank rear sleeve has this originally. Though I was told on good authority that most re builders open the 1/8" hole up to 5/32".
Thanks for the confirmation on the depth, Bernard.
Today I back peddled and went through the various model spec's in Blower. It actually states a dimension in the "J1-J2" listing. 7/64" deep.( 0.109") It doesn't give any such information in the other models. I now presume that the figure of 0.150" Bob Jones was referring to was for the smaller dia. cranks. I found that very confusing.
Tony, I too await your paper on bearing design. Sounds like a good one for document downloads.
I'd like to include Andrew Smith's earlier input from 26/11/2008 here, it's also relevant and informative.
http://triple-mregister.org/forums/topic.asp?TOPIC_ID=2213
Thanks to all.
Brian.
|
|
|
| |
Topic |
|
Forum Locked
Topic Locked
