Subject: Time:1:29 PM
OFFICE MEMO Grade 8 bolts on engine stand Date:12/9/93
>> I just went down
>> to the hardware store and bought some grade 8 bolts to mount the
>> engine to the engine stand. I still find it hard to believe that the
>> entire cantilevered weight of the engine can be held up by a couple of
>> 5/16" bolts, but I am putting my faith in those who have done so
>> without problems.
>I wonder if grade 8 is the best choice here. I know grade 8 bolts are
>very strong, but how well do they stand up to shear forces. They are on
>the brittle side. Is there a risk that a good whack on the engine
>somewhere might snap them off?
When I worked for International Harvester designing crawler tractors, they used
*only* Grade 8 bolts for the entire tractor. The word was that crawler tractors
have the hardest life of any type of construction equipment, nothing but
constant impacts and rough use in places like stone quarries. As far as I know,
bolt failures were reasonably rare.
Grade 8 bolts have an ultimate tensile strength of 150,000 psi, a proof load of
120,000 psi, and a recommended clamping load of 75% of proof load. For a
5/16-18 UNC bolt this translates to a 4720 lb. strength rating for a pure axial
load. Shear load strength is approximately the same.
I assume we're talking about the usual engine stand, with 4 swingable arms. So
for a 500 lb. engine with the center of mass 18" from the bell housing face, it
applies a bending moment about the pivot point (the two bottom arms) of 9000
in-lb. The vertical distance between the bottom and top arms is probably about
6", so the top bolts are seeing an axial load of say (9000/6/2) = 750 lb. each.
Shear load would be 250# each. Combined load 791#.
Thus even grade 2 bolts with a clamp load rating of 2160# would in theory be
enough, but bolts are cheaper than a dropped engine, and if it relieves your
anxiety, go for the stronger bolts. Above all keep that vertical distance
between the top and bottom arms big.
There are some counterfeit bolts on the market (i.e. don't meet the strength
criteria of the grade they are marked for). No guarantee your local hardware
store isn't selling any of those.
Just my opinions, and of course a standard liability disclaimer (you assume all
risk if you try anything). Your mileage may vary.
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