# Bearings for a belt redrive.

### Help Support HomeBuiltAirplanes.com:

#### Streffpilot

##### Well-Known Member
hey, I am looking at building a belt re-drive. just getting some ideas together at the moment. I was wondering what type of bearing to use up on the prop end. It will be a 14mm pitch x3 inch wide belt. With slack side tensioner. also, should I use a bearing on the engine crank side of the re-drive?

#### Jay Kempf

##### Curmudgeon in Training (CIT)
hey, I am looking at building a belt re-drive. just getting some ideas together at the moment. I was wondering what type of bearing to use up on the prop end. It will be a 14mm pitch x3 inch wide belt. With slack side tensioner. also, should I use a bearing on the engine crank side of the re-drive?
I am a big fan of deep groove ball bearings but you need to do some homework before you can decide on what bearing you need. There are so many things to consider including time before overhaul. If designing for say 200 hours with cheap parts and a little labor you can get away with a lot more than you can for 2000 hrs and big HP. With that belt dimension I am sure you aren't trying to hang onto 200 HP. Slack side tensioning is a good idea but it has to be setup correctly to not enhance big frequencies. Making a good guess with a lot of spring adjustability will allow tuning.

Bearing separation is key, People skimp too much on that. Then knowing the bearing DN is the next thing to design around meaning you need to know the input and output RPM of the system. If the engine is running at say 6000-10000 rpm that is the tricky bearing to pick and it might need and oil circulation system to have any life. The prop flange is probably going to be running at 3k rpm max so that is the easy one. But that one sees all the gyro loads.

The other big consideration in what you are designing is maintaining belt alignment. The two shafts need to be held very parallel and the assembly has to be stiff. So the farther they are away from each other the heavier the assembly.

If you have an idea what you are designing then some preliminary numbers will help tp point you in the right direction.

#### pictsidhe

##### Well-Known Member
Get some belt drive catalogues and look for datasheets and app notes. Gates was good last time I needed to do a belt drive.
For bearings, I like SKFs databook, it's got everything you need in it to size a rolling bearing. Mines some years old now, but
I'm sure they'll have kept it roughly the same, too **** useful.
I suspect prop loads are going to determine propshaft and bearing size.
As Jay said, deep groove ball for at least one prop bearing. A cylindrical roller bearing will do the other and stop you having to worry about differential expansion.
Whether you need a bearing on the engine side depends on your crank. Your average industrial engine is designed for belt loads, but car engines are not.

#### Streffpilot

##### Well-Known Member
max engine rpm is 5400. that is Take off. Im thinking 4600-4800 rpm cruise. puts out a max of 135 hp

Doppelposten

#### pictsidhe

##### Well-Known Member
Download the catalogues/databooks and go through the design procedures and you should have a good design. Wild guesses can be wrong, too small, stuff breaks, too big, extra cash and weight to haul around. You need more than rpm and power.
First thing to do is work out what sort of propshaft size you want and the gyro loads, The add belt stresses and then you can select the right belt and bearings.

#### Jay Kempf

##### Curmudgeon in Training (CIT)
max engine rpm is 5400. that is Take off. Im thinking 4600-4800 rpm cruise. puts out a max of 135 hp
At 5400 you may not be able to find a large axial/thrust load deep groove and you might need a somewhat small cross section bearing. You might also need to pay attention to loading and lubrication. These are all pretty standard design parameters. Where you have that condition you don't have the propeller with large gyro loads so it will all work out. Surprisingly prop thrust is a minor number in the mix.

#### pictsidhe

##### Well-Known Member
the engine bearing shouldn't need to take any thrust

#### stol

##### Well-Known Member
hey, I am looking at building a belt re-drive. just getting some ideas together at the moment. I was wondering what type of bearing to use up on the prop end. It will be a 14mm pitch x3 inch wide belt. With slack side tensioner. also, should I use a bearing on the engine crank side of the re-drive?

Absolutely...... IMHO..

#### ekimneirbo

##### Banned
What type of engine are you wanting to put the redrive on? What will be the total weight (hopefully) of the engine/redrive when complete? Air cooled/water cooled? What type of airplane do you want to put it in?

#### Streffpilot

##### Well-Known Member
Well like i said, I'm just kicking around ideas. total fwf goal is under 200 lbs. will be a mercury inline 6 2 stroke. On a Dakota Hawk. Like i said. just spitballing ideas.

#### BillM

I was thinking that the wheel bearing from the front wheel of a Dodge Caravan should be about perfect. It is sealed, pre-lubricated and cheap (about $50.00). It even has a flange for mounting. What would be the issue for NOT using it? BillM #### clanon ##### Well-Known Member Well , the SKF site mostly guide you through ,i think it will be : loads (axial;radial;torque) vibration and max RPM (being the most important ones). Then reliability (through safe max RPM and lubrication) #### ekimneirbo ##### Banned Well like i said, I'm just kicking around ideas. total fwf goal is under 200 lbs. will be a mercury inline 6 2 stroke. On a Dakota Hawk. Like i said. just spitballing ideas. Everyone has there own preferences, but I would recommend something with 4 strokes.........you might want to take a look at the Corvairs with the 5th bearing. #### Jay Kempf ##### Curmudgeon in Training (CIT) Lifetime Supporter I have thought about this issue for a long while and have a question- I was thinking that the wheel bearing from the front wheel of a Dodge Caravan should be about perfect. It is sealed, pre-lubricated and cheap (about$50.00). It even has a flange for mounting.
What would be the issue for NOT using it?
BillM
Props turn 2500 or more. The average sized car wheel turns say 1100-1200 RPM at 75 MPH rotating 35-50 pounds at something like 26" diameter max. The prop is normally twice that diameter with a lot of the weight way out in radius. The gyro forces of the prop are more difficult to design around plus the impulses and the constant rate of rotation trusting your life to it. Despite the pounding that most people think tires are taking they are suspension and tire damped softening the blows in time. 100k miles at 75 MPH is about 1300 hours of life and at 1100 RPM that is something like 88M cycles. No car runs its life out running 75 MPH getting pounded in pot holes. A plane on the other hand spends say half it's life at around 75% power at around 2500 RPM for say 1500 hours TBO. 1500 hours at 2500 RPM is 225M cycles. So you do the comparison. 88M vs. 225M vs 1100 RPM vs. 2500 RPM. Those are orders of magnitude different life constants to factor in.

Most times when you run the numbers a bearing needs to be continuously lubed to survive in a prop hub for any amount of time. Time on the other hand is a life variable that you can play with. When doing the same numbers on the prop hub you get HUGE numbers if you have a large reduction ratio.

How often does that car hub make it all they way to 100k miles to the point where you would trust your life to it?

#### Streffpilot

##### Well-Known Member
Everyone has there own preferences, but I would
recommend something with 4 strokes.........you might want to take a look at the Corvairs with the 5th bearing.

I am looking at that as an option. i just don't like the weight. Plus, without a re-drive, the max they are getting is about 100 hp.

Plus, I'v got a 115 hp Merc sitting on the back of a junk boat.......so it put me to thinkin'......never good

#### Jay Kempf

##### Curmudgeon in Training (CIT)
I am looking at that as an option. i just don't like the weight. Plus, without a re-drive, the max they are getting is about 100 hp.

Plus, I'v got a 115 hp Merc sitting on the back of a junk boat.......so it put me to thinkin'......never good
Go ahead and explore that 6 cyl Merc. It's a good idea. Someone put a modern V6 outboard on the front of a biplane. That install looks good. Somewhere here there is a thread on it.

#### Streffpilot

##### Well-Known Member
Go ahead and explore that 6 cyl Merc. It's a good idea. Someone put a modern V6 outboard on the front of a biplane. That install looks good. Somewhere here there is a thread on it.
The good thing about the Merc is it is light.

#### pictsidhe

##### Well-Known Member
Outboard engines are definitely worth looking at, good power to weight ratio, built to be held wide open continuously. Bit thirsty, but you can't have everything. A six cylinder two stroke won't have much torque fluctuation; easier on redrives.
A redrive needs to done carefully. It shouldn't take more than a few days work to have a solid design that's not far from optimum weight, if you work on design parameters established by others.
While the junkyard approach can be useful, it's probably going to be heavy compared to a custom solution. You may well find that an XYZ car wheel bearing happens to be just the right size. But you need to work out the loads and what size bearing you need first, or it's lucky dip. Could be solid but way heavier than needed, or you may lose your prop at the end of the runway...
If you don't want to work it out, I'd highly recommend reusing a design that has been proven. Or just buying one from a redrive guy.