I was looking through the threads reading all the advice for Caddy DD Props and stuff. I know it will depend on a number of different factors but what would be considered good preventive maintainance as far as replacing the rear main bearing. Any history as far a breaking Caddy Cranks?
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How long do Caddy DD Rear Mains Last!
- Thread starter COLD
- Start date
bondsman
Well-known member
i have heard of more caddy's breaking there cheep flex plates more than a crankshaft.one of the crazy caddy builders by the name of richard potter has turned his one caddy motor to 8000rpm numerous times racing with a stock crank as long as you keep it balanced you should not have a problem with a crank. as far as my boat when it was dd i went 130hours until i went gearbox . my rear seal never failed.and center main thrust bearings only were down so far since there is a keeper on the crank that will only let it travel so far but you still after awhile will get crank end play. here is a great caddy link for you cold eh. all the caddy info you can digest. http://www.cowboyseven.us/forum/index.php
Perfect! Another forum to look through. My wife will love that! I just saw the pics of the Buick 455 that broke it's crank and sunk and wondered. Saw an add for a 1970 Elderado with a 500 that supposed to be just rebuilt before the car sat a few years, WEEL SEE! I thought DD Caddy might be most inexpensive way in if my Machine Shop Buddy is to busy to help build a drive. They do sound like fun!
jdotson
Well-known member
I put about 230 hours on a direct drive and the bearings looked fine when I tore it down. The engine was still running fine but I was switching to reduction and needed the crank for my other build up.
Bondsman I can attest to the flex plate problems. I went through three of them in about a four month period and it took the nose off the starter every time. Balancing the rotating assembly is a must anytime any part of the assembly is changed.
Bondsman I can attest to the flex plate problems. I went through three of them in about a four month period and it took the nose off the starter every time. Balancing the rotating assembly is a must anytime any part of the assembly is changed.
With any DD automotive engine I would hazard a guess the thrust collar/washer/bearings will wear faster then the mains will. Caddy is a tough ole bird and I dont think I'd worry too muchg about the rear mains unless there is already something making you suspicious.
Most of my building experience is Buicks but I have a fairly high opinion of the Caddy being relatively trouble free when set up right.
Scotty
Most of my building experience is Buicks but I have a fairly high opinion of the Caddy being relatively trouble free when set up right.
Scotty
The thrust is the one I was concerned about. From what I read from bondsman's post above the thrust is in the center position on the caddy unlike the chevys in the rear position. The thrust bearing is usually a part of the main throw bearing assembly, while I have worked on some stuff that has a 4 peice bearing for one position. Do you think its balancing cracking the flexplates? They were designed to be bolted to a torque converter and that would stiffen everything up. Are caddy flexplates a larger diameter than the two chevy choices with more big holes? I have run across cracked ones even bolted to torques. Something to keep an eye on for sure, Thanks!
The cadi rear main is about the strongest thing alive how ever i do know of three cases were the crankshaft broke at the #7&8 rod journal taking out the back end of the block.All three invovled the use of 2 blade powershift props. one of these boats the motor had just been rebuilt like 1 hour run time.the boat was opperating at 2000 rpm in a strait line when the motor let go taking the back of the block out,and by the time it was over the prop was about 2 inches long and the hub was history. Luckily all of the destruction was confined to the upper and left side of the cage. internaly all the engine parts checked out fine matter of fact the rods and pistons are currently being run in southern style.So what could cause this damaged ?
1 A thing they call gyroscopic moment (the blades of a propeller are not loaded the same) Hopefuly some one will post a better discription of what a gyroscopic moment is.
2 Prop not set right.
1 A thing they call gyroscopic moment (the blades of a propeller are not loaded the same) Hopefuly some one will post a better discription of what a gyroscopic moment is.
2 Prop not set right.
Waterthunder
Well-known member
One thing that creeps me out with the Caddy’s and some other motors. The crank was never intended for a forward thrust it’s amazing that they live at all. When the thrust bearing is in the rear of the crank all the force is controlled at the rear of the crank and is not transmitted thru out the bottom end especially the crankshaft. When the thrust bearing is located in the middle main the forward thrust travels half way thru the crank shaft, it’s amazing that these cranks survive. There’s already harmonics and twisting occurring in motors then we add the forward thrust of a prop on top of everything else. Plus the fact the front half of the crank is operating with no forward thrust and the rear half is absorbing all the forward thrust. DD car motors are fortunate they make little horsepower and operate below 3,500RPM’s I think this is the only reason they live.
sorry if this is a little off thread just thought I would mention it.the only other crankshaft brake that I seen was on a dd 350 , it had broke right at the rear main at the crank flange ,although the prop and cage was history, the engine ran fine just could'nt crank it after it was shut down it was running a 68 x 34 paddle prop.
I agree Waterthunder, thats why i'm asking. But if it works without crank breakage problems it works for me! If the thrust bearing is lasting a reasonable time I don't think I would even put an external thrust off the front of the crank, might compound problems. It would be nice to have a thrust at the prop end but that would be a little more work. I'll look into it if I buy this thing! :wink:
Waterthunder
Well-known member
If you are building the engine yourself I can tell you a neat little trick that will double the life of your thrust bearings. P.M. me for details it’s very simple and only takes ten minutes to do plus it doesn’t cost anything. I learned this from building engines for drag cars that run standard transmissions and adopted it to DD airboat motors.
Gyroscopic precession (Not precision) is an effect normally associated with roateryblades in hellicopter and autogyros, however the post above about it is generally correct about unequal loading of the blades.
A propeller is two wings each flying in a circle. Imagine that, as in a hellicopter, you are going forward and the big blade is rotating counter clockwise. The side of the craft where the blade is moving forward the blade is traveling at a speed of the prop RPM plus the craft forward speed. On the opposite side of the craft the propeller blade is traveling at the crafts forward speed minus the props RPM. This causes a big difference in lift ability between the two sides.
now put that prop on an airboat and move the boat vertically as in going over waves and you get a degree of the same effect, slide the boat to one side and again you have an unequal load on the prop.
I would be truely shocked if this can make any significant strucural difference on an airboat prop since the amount is precession is tiny.
Moment, however, is a condition where you imnagine a lever and fulcrum etc, the lever is the prop and has two different length or pitched blades. As RPM increases the moment or bending force becomes greater and the unbalance induces shake or vibration. I CAN envision this as progressing to the destruction of the prop and engine in fairly short order. It has always been important in airboats to have props ballanced and tracked. Having said this, in emergency situations we have all probably seen folks limp in with a tip missing or split open. It's less likely to limp in sucessfully with a piece of the structure of the prop missing.
If a prop manufacturer was sending out blades of different weights or airfoil contours in the same kit, it sure could cause the destruction of the engine and the prop both, and of course the evidence it was or was not the prop that caused it. The same can be said for pitching the blades, both have to be the SAME. Im envisioning a situation where maybe the blades were pitched properly but the hub bolts werent fully tightened to the correct torque and they worked loose enough to allow one of the blades to rotate in the hub.
When we buy a new prop it makes sense to weigh and measure things just to double check that what you got is what you ordered. Especially if there is suspicion raised with the product before hand. Most manufacturers have some degree of Quality Control to prevent this, but when were dealing with humans, the pissibities for error are limitless.
In the days when all our props were trees this never came up. Besides they looked good too
Scotty
A propeller is two wings each flying in a circle. Imagine that, as in a hellicopter, you are going forward and the big blade is rotating counter clockwise. The side of the craft where the blade is moving forward the blade is traveling at a speed of the prop RPM plus the craft forward speed. On the opposite side of the craft the propeller blade is traveling at the crafts forward speed minus the props RPM. This causes a big difference in lift ability between the two sides.
now put that prop on an airboat and move the boat vertically as in going over waves and you get a degree of the same effect, slide the boat to one side and again you have an unequal load on the prop.
I would be truely shocked if this can make any significant strucural difference on an airboat prop since the amount is precession is tiny.
Moment, however, is a condition where you imnagine a lever and fulcrum etc, the lever is the prop and has two different length or pitched blades. As RPM increases the moment or bending force becomes greater and the unbalance induces shake or vibration. I CAN envision this as progressing to the destruction of the prop and engine in fairly short order. It has always been important in airboats to have props ballanced and tracked. Having said this, in emergency situations we have all probably seen folks limp in with a tip missing or split open. It's less likely to limp in sucessfully with a piece of the structure of the prop missing.
If a prop manufacturer was sending out blades of different weights or airfoil contours in the same kit, it sure could cause the destruction of the engine and the prop both, and of course the evidence it was or was not the prop that caused it. The same can be said for pitching the blades, both have to be the SAME. Im envisioning a situation where maybe the blades were pitched properly but the hub bolts werent fully tightened to the correct torque and they worked loose enough to allow one of the blades to rotate in the hub.
When we buy a new prop it makes sense to weigh and measure things just to double check that what you got is what you ordered. Especially if there is suspicion raised with the product before hand. Most manufacturers have some degree of Quality Control to prevent this, but when were dealing with humans, the pissibities for error are limitless.
In the days when all our props were trees this never came up. Besides they looked good too
Scotty
Waterthunder
Well-known member
OOP's
Gyroscopic precession:
It seems like we have discussed this before, but maybe not. I found this neat little gif which helps explain it. One end of the toy gyro is sitting on a stand and there is a weight on the other. At first glance it would seem that the weight would push that end of the gyro down. But, gyroscopic precession says that a spinng mass will react to any out side force applied 90 degrees later. Therfore the gyro spins when the down force is aplied.
gif credit to "Siltec"
If you restrict the motion of the gyro, the the little axle is going to see some bending loads as the gyro tries to turn and can't.
An airboat propeller is a gyro, the bigger it is or the faster it turns the greater the forces involved. When the boat turns especially in a spin the gyroscopic forces will try to either push the bow up or down depending on the direction of the turn and the rotation of the prop. In the process ther are big bending loads applied to the propeller blades and the crankshaft. The forces are trying to bend the blades and the crank back and forth at the same frequency and the prop speed.
This is like bending a peice of solid wire back and forth: it's live will be limited.
These forces are so great that a typical O540 powered aerobatic airplane with a METAL prop can break a crank if used in manuevers that have high yaw or pitch rates. There a cases of 540 cranks letting go at around 100 hours. For that reason, most of the aerobatic planes have switched to composite props. The props are lighter and produce less gyroscopic forces.
jim
It seems like we have discussed this before, but maybe not. I found this neat little gif which helps explain it. One end of the toy gyro is sitting on a stand and there is a weight on the other. At first glance it would seem that the weight would push that end of the gyro down. But, gyroscopic precession says that a spinng mass will react to any out side force applied 90 degrees later. Therfore the gyro spins when the down force is aplied.
If you restrict the motion of the gyro, the the little axle is going to see some bending loads as the gyro tries to turn and can't.
An airboat propeller is a gyro, the bigger it is or the faster it turns the greater the forces involved. When the boat turns especially in a spin the gyroscopic forces will try to either push the bow up or down depending on the direction of the turn and the rotation of the prop. In the process ther are big bending loads applied to the propeller blades and the crankshaft. The forces are trying to bend the blades and the crank back and forth at the same frequency and the prop speed.
This is like bending a peice of solid wire back and forth: it's live will be limited.
These forces are so great that a typical O540 powered aerobatic airplane with a METAL prop can break a crank if used in manuevers that have high yaw or pitch rates. There a cases of 540 cranks letting go at around 100 hours. For that reason, most of the aerobatic planes have switched to composite props. The props are lighter and produce less gyroscopic forces.
jim
Good post Jim, I get so used to discussing it in terms of advancing and retreating blades in other forums I belong to, I neglected the 90 degree component. In rotorcraft it manifests itself in blade flap. And that component is what actually appies to the question asked. My bad. Yup that'll do it.
http://www.rotaryforum.com/forum has excellent archives from aircraft design engineers for those who want to go really deep into the subject.
Being a function of physics, there is no escape from it, just a degree of mitigation.
Sure glad we dont have to carry parachutes on airboats ! Though I have seen a few that could use drag chutes to get em slowed down hahahahah
Scotty
http://www.rotaryforum.com/forum has excellent archives from aircraft design engineers for those who want to go really deep into the subject.
Being a function of physics, there is no escape from it, just a degree of mitigation.
Sure glad we dont have to carry parachutes on airboats ! Though I have seen a few that could use drag chutes to get em slowed down hahahahah
Scotty