743 Humming Sound

Help Support SkidSteer Forum:

Tazza

Administrator
Staff member
Admin
Joined
Dec 7, 2004
Messages
16,697
Ok, this may sound stupid, but my 743 has always had this humming sound that sounds like its coming from the hydro pump. If i move the levers forward and back it stops then comes back. I thought it may have been the by-pass valve in the hydro filter so i changed the filter. Yes yes, i should have changed it as soon as i got it, the old filter was dated september 95!!!!! Even after the filter change its the same. Has anyone had this problem before? the only place i haven't looked was the port block, the bit that joins to the oil cooler. When its making this noise, you can grab the hydro hose that goes to the filter and you can feel the vibrations, but these can be coming from anywhere...
The machine still runs fine, its just a little anoying. Put the revs up high and you can't hear it.
 

ajwgator

Well-known member
Joined
Oct 12, 2005
Messages
58
Well Tazza, not having any experience with a 743, I can only rely on the years of aircraft hydraulics which is pretty close to the way our machines are designed. That being said take all of this as information and bear with me because I know you probably know some of this. Humming sounds almost always come from something that is moving all the time and is actually a vibration of some sort. So looking at the way things work in the flow schematics of my 843 (and I think all the bobcats) while running and when every thing is sitting still and all controls in neutral, the only thing running all the time are the two pumps and gear pump. All the hydraulic fluid, while even under presssure, is just running in a circle. We are looking at constant volume pumps (front & rear) that supply hydraulic pressure to the motors that drive the wheels. And on the front of the pumps is the gear pump that supplies the pressure for the tilt & lift cylinders. The gear pump also supplies the hydraulic fliud to the inlet side of the front & rear pumps. When everything (levers & foot controls) is in neutral the shafts and variable pistion assemlbies in the hydro pumps, and the gears in the gear pump are the only things that are turning. The speed they rotate of course depends on the engine RPM. The only things that I can see that would or could produce a humming or vibration in there would be the thrush bearings or the shaft bearings in the hydraulic pumps. I lean more to the thrust bearings because when things are in neutral there are not any forces being applied to that bearing. The shaft bearings have the same forces applied to them all the time in neutral or not. Going back to the thrust bearings now, you said when you move the levers the humming stops but comes back when you go back to neutral. Looking at that action, we are moving the floating plate that applies pressure to the rotating variable piston assembly in the hydro pump. That action pushes or pulls the internal assembly a little that is being driven by the pump shaft and in turn is applying forces against the thrust bearing. That force against the thrust bearing stops the vibration until things go back to neutral. Bypass valves, port valves, relief valves, and the like are just that. Valves are either open or closed either manually or with designed spring pressure and thus will not vibrate. I know some people say that a valve will "vibrate" because it is opening and closing all the time very fast. That is not really the case and the noise one hears when a valve actuates is only the fluid under pressure traveling though it. All in all, I don't belive there is any wrong in the hydro pumps other than just a little wear and tear on someting that has to turn constantly by design. In my opinion it is not worth tearing it down to replace the thrust bearing unless you have to for another reason. Hope this provides a little more insight to it all.
 

Tazza

Administrator
Staff member
Admin
Joined
Dec 7, 2004
Messages
16,697
Well Tazza, not having any experience with a 743, I can only rely on the years of aircraft hydraulics which is pretty close to the way our machines are designed. That being said take all of this as information and bear with me because I know you probably know some of this. Humming sounds almost always come from something that is moving all the time and is actually a vibration of some sort. So looking at the way things work in the flow schematics of my 843 (and I think all the bobcats) while running and when every thing is sitting still and all controls in neutral, the only thing running all the time are the two pumps and gear pump. All the hydraulic fluid, while even under presssure, is just running in a circle. We are looking at constant volume pumps (front & rear) that supply hydraulic pressure to the motors that drive the wheels. And on the front of the pumps is the gear pump that supplies the pressure for the tilt & lift cylinders. The gear pump also supplies the hydraulic fliud to the inlet side of the front & rear pumps. When everything (levers & foot controls) is in neutral the shafts and variable pistion assemlbies in the hydro pumps, and the gears in the gear pump are the only things that are turning. The speed they rotate of course depends on the engine RPM. The only things that I can see that would or could produce a humming or vibration in there would be the thrush bearings or the shaft bearings in the hydraulic pumps. I lean more to the thrust bearings because when things are in neutral there are not any forces being applied to that bearing. The shaft bearings have the same forces applied to them all the time in neutral or not. Going back to the thrust bearings now, you said when you move the levers the humming stops but comes back when you go back to neutral. Looking at that action, we are moving the floating plate that applies pressure to the rotating variable piston assembly in the hydro pump. That action pushes or pulls the internal assembly a little that is being driven by the pump shaft and in turn is applying forces against the thrust bearing. That force against the thrust bearing stops the vibration until things go back to neutral. Bypass valves, port valves, relief valves, and the like are just that. Valves are either open or closed either manually or with designed spring pressure and thus will not vibrate. I know some people say that a valve will "vibrate" because it is opening and closing all the time very fast. That is not really the case and the noise one hears when a valve actuates is only the fluid under pressure traveling though it. All in all, I don't belive there is any wrong in the hydro pumps other than just a little wear and tear on someting that has to turn constantly by design. In my opinion it is not worth tearing it down to replace the thrust bearing unless you have to for another reason. Hope this provides a little more insight to it all.
I don't remember any thrust bearings inside the hydro pump, there were cup and cone style ones though, front and back.
The humming is indeed vibrations, when i move the steering levers to neutral its fine for a while then starts some times. It doesn't seem to be a bearing noise as its not constant when in neutral. Even if it is a bearing, i'm not going to fiddle with it again, its too much effort to remove then replace the pump. I'm going to sell this machine when i get my other one all finished.
With you having experience with aircraft hydraulics i bet it helps you with any problems you encounter.
Thanks for the reply
 

ajwgator

Well-known member
Joined
Oct 12, 2005
Messages
58
I don't remember any thrust bearings inside the hydro pump, there were cup and cone style ones though, front and back.
The humming is indeed vibrations, when i move the steering levers to neutral its fine for a while then starts some times. It doesn't seem to be a bearing noise as its not constant when in neutral. Even if it is a bearing, i'm not going to fiddle with it again, its too much effort to remove then replace the pump. I'm going to sell this machine when i get my other one all finished.
With you having experience with aircraft hydraulics i bet it helps you with any problems you encounter.
Thanks for the reply
Your welcome, and yes the hydraulic experience comes in handy but I'm never too old to learn something new...everyday. As I said no experience with a 743 and I was assuming the pumps were close to the same. You know what "assume" means don't you! It makes an "ass" out of "u & me"! Oh well, sorry I wasn't much help. I appriecate all your inputs to the forum. By the way, how do get the space between your paragraphs? When I try and do that in my replies is look ok but when I hit post it all shows up as one big paragraph. Just wondering.
 

Tazza

Administrator
Staff member
Admin
Joined
Dec 7, 2004
Messages
16,697
Your welcome, and yes the hydraulic experience comes in handy but I'm never too old to learn something new...everyday. As I said no experience with a 743 and I was assuming the pumps were close to the same. You know what "assume" means don't you! It makes an "ass" out of "u & me"! Oh well, sorry I wasn't much help. I appriecate all your inputs to the forum. By the way, how do get the space between your paragraphs? When I try and do that in my replies is look ok but when I hit post it all shows up as one big paragraph. Just wondering.
You go into your profile and you need to set it to use HTML editor i think it was.
I had the exact same problem when i just started wondering around here.
See if thats it, if not i will look up what i did for ya :)
 

goodtech

Well-known member
Joined
Nov 22, 2005
Messages
112
You go into your profile and you need to set it to use HTML editor i think it was.
I had the exact same problem when i just started wondering around here.
See if thats it, if not i will look up what i did for ya :)
Tazz you need to adjust your steering it is out a little bit. what it is doing is putting a load on your pump set, causing the vibration, and you may want to check your driveline, I have had a couple have a vibration from that being really loose. Good luck
 

Tazza

Administrator
Staff member
Admin
Joined
Dec 7, 2004
Messages
16,697
Tazz you need to adjust your steering it is out a little bit. what it is doing is putting a load on your pump set, causing the vibration, and you may want to check your driveline, I have had a couple have a vibration from that being really loose. Good luck
ooh how i have tried to get it rite....
I just can't get it there, it works well then i push the levers back and forth and the F****** thing is still slightly out, so when i start and stop the machine i kick the brake off. I will give it another go at getting it rite, i need to buy a new set of cams for the steering and i really don't like how much slop is on the nylon guides. I bought them new off bobcat and they seem to move around a bit too much. For the one i an doing up, i am getting a friend to cut slots into a piece of nylon for me so i can make my own, these will have minimal play. I think i may need to tension up the centring spring too, i just don't think its pulling it back to centre.
 
Top