2003 Cat 236 skid steer, 3000 hrs i rebuilt the bucket tilt cylinders last year. I didnt have to bleed them at all and the rebuild fixed the cylinder leak down issue accordingly, i rebuilt the lift cylinders this year, hoping to fix issue of the lift arms settling down over time but instead it didnt fix the issue, it got worse. im guessing i have air in the system, ive tried bleeding the lines by loosening them at the cylinder and collecting the oil that comes out, but that didnt help. is there a special bleeding method to use? ive tried cycling the lift arms up and down 30 times probably trying to get air out thanks -phil
I think it is highly unlikely to be the cylinders causing a steady leak down of the lift arms. A small initial drop yes, but not a continuous fall. See the diagram below. The oil below the piston is the full capacity of the cylinder barrel (diameter 2 - 2.56"), above the piston it is the barrel minus the diameter of the rod (3 - 1.77"). With hydraulic oil being basically an incompressible fluid if the cylinder moves down say 1" (with the control valve spool in the closed position) where does the extra oil go.? There isn't enough space for it above the piston because most of the cylinder barrel is occupied by the rod. If the piston moves down 1" it will displace 5.15 cu.in. of oil. In the same 1" of movement there will be only 2.80 cu.in. of space available above the piston for that oil to go.
right ive read this before, but i replaced the seals anyway just in case. one seal was broken completely inside so it needed it. i have an initial drop after lifting the arm, and then a steady slow sinking. the initial drop wasnt there before the rebuild, so im guessing i have air stuck inside. and then the steady low leakdown i know is normal to some extent, but im hoping to be able to fix it. how fast the arms sink depends on the weight in the bucket as well.
To exercise any entrained air out of the cylinders start by getting them in the mid-point of the stroke and with the engine at Low Idle and the oil at operating temperature. Start by moving the cylinders a couple of inches in each direction and back to mid-point again. Increase the distance you move the cylinders a small amount at a time over 4-5 steps but never to full stroke. Finally slowly take the cylinders from mid-point to full stroke in each direction and back to mid-point. If there is any air in the cylinders that ought to get it out. I would think that the oil is getting back to tank via the (allegedly closed) control valve TBH.
Port reliefs and load checks in the control valve (if it has them) are the usual suspects. CAT liked to use something called "Resolvers" in some of their systems, damned nightmare as far as I could tell, but probably not on this small a machine?
Not saying this is the issue, just an observation from experience. Often, when I'm called to a customer to diagnose a hydraulic complaint where one thing has been recently fixed and now something else related is malfunctioning, I find a contamination issue. This is related to a dirty repair. Especially with hydraulic hoses. Ports don't get capped, new hoses don't get cleaned before install, or dirt gets packed into unsealed components during install. (Side note) The average laborer, or operator that often gets tasked with a hyd. hose change doesn't have the knowledge of contamination control and can mess up a lot of stuff. Next, the repair can go sideways when the hydraulic reservoir is filled or topped off. Dirty funnels, dirty buckets, dirty oil. Folks don't understand how clean it needs to be. The ISO 4406 Cat roll-off spec is 18/15 for your 236. Like Iantraxco said, you will probably find a piece of crud inside a P/O check, or the port relief relief.
[QUOTE="Coaldust]Next, the repair can go sideways when the hydraulic reservoir is filled or topped off. Dirty funnels, dirty buckets, dirty oil. Folks don't understand how clean it needs to be. The ISO 4406 Cat roll-off spec is 18/15 for your 236.[/QUOTE] The roll-off spec is now 16/13, the same as the fill oil spec. Those 789s I just put together were arriving with “Certified Clean Machine” tags with ISO Numbers in factory-filled systems as low as 10/8.......
Thanks Nige! 18/15 has been burnt into my brain. 10/8? Wow. Everyone thinks I’m crazy when I mention new hydraulic oil needs filtered before going in the machine. And not with a rag or coffee filter. Lol Ain’t nobody gots time fo dat. It’s new oil. It must be clean. It’s new oil. It’s in a shiny blue drum.
Pistons didn’t get put back on backward did they? Fixed a machine with similar issues and discovered that was the culprit. I’m inclined to it’s trash somewhere as well, I just wonder about cylinder assembly since it got worse after that change. I’ve never had any problems with purging air after a cylinder repair or hose change.
Often with leaking cylinders, folks wait too long to fix them. The piston wear band material chunks off, pistons scraping the barrel make metal, chunks of u-cup seals and buffer rings. Where does all that junk go? The cylinders get replaced or repaired but there is now plenty of contaminants to mess up the works. The contamination may not cause immediate issues. Or, the customer may think the new cylinders are the problem and point blame at the shop that repaired them. Hydraulic repairs can escalate into crazy -expensive projects in a heartbeat. Preventative MX is key.
Coatdust, thanks for the comments! ive had the Cat 236 for about 5 years. the lift arms had always sank down , i figured i needed cylinder rebuilds, but was just now getting around to do it finally since it didn't really effect overall operation that much. i have rebuilt front tilt cylinders without issue, and it fixed the tilt cylinder leak down issues that i was having (or at least greatly reduced it to the point i didn't notice it anymore). i rebuilt the lift cylinders the same way, being very careful to get everything back together the way i took it apart. i did have a broken seal on one lift piston, not sure if there was chunks left over inside. i looked down the cylinder bore and didn't see anything, but of course it could be in the line or at a valve somewhere causing issues. the overall leakdown rate of the lift arms is about the same as before the bebuild, but there is an initial quick drop after i stop raising the lift arms. so with a bucket full of dirt, when i return the joystick from lift to neutral, the lift piston immediately sinks back into the cylinder about 3/4 of an inch over 4 seconds, and bucket drops accordingly. and this DIDNT happen as much before i tried to do the rebuild. it happened a little but its way worse now. then the sink rate slows down dramatically and the bucket sinks at the rate of 1in every 30 seconds (from measuring with a tape measure and stop watch). thats about the same as it was before i rebuilt the lift cylinder
also this is the first hydraulic machine ive owned, so i dont know what noises are normal. 1. for this machine, when the bucket tilt or lift pistons reach the end of their travel, and hit the stops, if the joystick isnt returned to neutral position, the engine starts chugging, like its overloaded. i know if there is an overpressure situation in this case, there are internal relief valves that open fluid bypasses, but im not sure if it should be more smooth than this, or if a chugging engine sound is normal 2. during normal bucket movements, there is no noise from the hydraulic pump components, i just hear the engine running. however when i steer left or right however (left side wheels going one direction, right side wheels going other direction), i hear a little bit of hydraulic pump whining noise. not sure if this is normal or not thanks
The lugging down noise the engine makes when the cylinder pistons hit the stops is not normal. At that point the relief valve should open and you ought to hear the unmistakable noise of the hydraulics as the oil bypasses back to the tank. Maybe the next step ought to be checking all the pressures - pilot, main relief, & line reliefs, as a minimum.
I agree with Nige. With the additional information provided, I don’t think the lift cylinders were the problem. It’s time to cut open hyd filters and check pressures.
I did my best to keep it clean. i cleaned off all the line connections before disconnecting them so that there wouldnt be excess dirt in the area once disconnected. right after disconnecting lines i wrapped electrical tape around the ends of the fittings so keep dirt out empty cylinders got capped with clean rags and tape i made sure everything was clean and oiled before reinserting the pistons
im new to hydraulics, so ill get a cat 236 specific repair manual and study up, do some tests and report back. i imagine everything is serviceable without specialized expensive tools? most likely i just need to replace seals or clean lines and valves out?
Post a Serial Number, or at least a S/N prefix, and I’ll look up the Publication reference of the Manual to suit your machine. Service Manuals aren’t cheap, I’ll warn you of that now. Have you changed the hydraulic system filter recently.? If not I suggest that you do, and then do a filter cut on the old one looking for particles. Some good videos by a YouTuber called Adept Ape showing how to do it.
wow thanks for all the help! serial number: CAT00236A4YZ04598 all filters changed last year at 3000 hrs i follow adept ape also on youtube
Is drift the same engine ON vs engine OFF?
i havnt measured exactly yet, but from using the machine, im pretty the arms sink at the same rate regardless if engine is off or on.
The Cat Publication reference for a Service Manual for your machine is RENR4830. Here's one - https://www.ebay.com/itm/RENR4830-N...er-Loader-Repair-Service-Manual-/311885409472 New in paper form the list price is $665. You can get the same manual on CD for $365.
ok thanks ill get it. also is there a general consensus on the best aftermarket hydraulic oil recommendation for these cat machines? the dealer wants $18/quart some people are using 10w30 motor oil instead ($3/quart or less) and ive found a general hydraulic oil on walmart.com, 4.75 gallons for $30 ($1.60 /quart). called "Sinopec L-HM 32"
If you want to get the CD version containing the same info here’s the link - https://www.servicemanualpro.com/?page=Products&searchbox=4YZ04598 That Sinopec hydraulic oil seems to get good reviews from others on here.
The CAT dealer I worked at had an issue with high numbers coming back consistently on machines they were contracted to service for the customers. Somebody finally suggested sampling the oil coming out of the service trucks, then the bulk tanks at the dealer, then the supplier's trucks, then his tanks, etc. All dirty, with shiny fresh from the tank farm oils. Better pumps and 25/10/3/3 medium pressure filters daisy chained at the dealership and something similar on the lube trucks solved that problem, but ***? eh? Oh, BTW, from one oil company I bought from, if you went by their yard you could watch them fill drums and 5 gallon buckets with various products, from their tank trucks which they filled from the bulk tanks with overhead drops. Wind blowing dust everywhere. Cleanliness is next to... impossible?
"the overall leakdown rate of the lift arms is about the same as before the bebuild, but there is an initial quick drop after i stop raising the lift arms. so with a bucket full of dirt, when i return the joystick from lift to neutral, the lift piston immediately sinks back into the cylinder about 3/4 of an inch over 4 seconds, and bucket drops accordingly. and this DIDNT happen as much before i tried to do the rebuild. it happened a little but its way worse now." This sounds to me, and bear with me, like you're piston(s) is/are bypassing enough to put your cylinders into displacement mode, in other words the piston packing is still leaking a little, so the rod packing takes all the pressure, and it will be higher than normal due to the small size of the rod, which is acting as the cylinder piston. Still going with that assumption and the fact you said one old piston seal was broken, it probably settled less at first because the broken seal had the cylinders acting in displacement mode basically all the time.
[QUOTE="This sounds to me, and bear with me, like you're piston(s) is/are bypassing enough to put your cylinders into displacement mode, in other words the piston packing is still leaking a little, so the rod packing takes all the pressure, and it will be higher than normal due to the small size of the rod, which is acting as the cylinder piston. Still going with that assumption and the fact you said one old piston seal was broken, it probably settled less at first because the broken seal had the cylinders acting in displacement mode basically all the time.[/QUOTE] interesting theory. so correlated side note: when i installed the tilt pistons, they were really hard to get out and get back in. BUT when i installed the lift pistons, the ones in question, they came out super easy (by hand, no tools required) and went went back in super easy. i was really surprised. a loose fit would create the effect you are referring to i believe. so i bought this machine used, in Miami (my first mistake), where the cubans here are well known for jerry rigging everything to save a few dollars. the drive joystick is installed backwards on it. someone working on it didnt know what they were doing. the oil seals on the lift arms had been replaced already, so someone before me already had had the lift cylinders apart before i bought it. all the main oil seal that keeps oil from leaking past the piston was not spec, it was a different color than what CAT supplied. and one was broken in half. im wondering if someone didnt swap out damaged lift pistons in this machine for some other wrong size lift piston and call it a day? is that even possible? on the other hand, both of the lift pistons were loose like this. and this is just me guessing why the lift pistons would be not tight. i bought the new lift seal kit directly from Cat. and the other rings and seals appeared all to be the same as the ones i was replacing. maybe there are size specs i can look up, and then measure the diameters on the lift piston parts i have and double check to make sure i have the right piston installed. why else would a piston be loose like that right after a rebuild?
Why would a Piston be loose? Here is a possibility. Slim, but worth noting. Caterpillar sells oversized remanufactured cylinders and offers oversized seal kits. Which allow you to hone a scratched cylinder oversized to salvage it. Maybe your lift cylinders are oversized and you used a standard size seal kit? My Pro Tip for the day ( no charge, totally free!) Measure cylinder bore dia. before ordering parts.
It's also possible something simple like an internal O-ring on the piston to rod was missed or damaged on installation.
Have you checked this valve
wow, that is a pro tip! never would have thought of that.
Are there any examples to show of Cat oversize seals for a specific cylinder(s)?
Welder Dave, The Cat publication PEKP1020-08 or newer, Hydraulic Cylinder And Seal Reference Guide is a handy book to keep around. Not sure what the newest edition is. I'm a little out of the loop. It's been a dealer only publication. The general idea is you can go up to about .014" over-size and still use the original size seal kit, then you can go to .030" and lastly to .060". Bolt on heads with oval head seals sometimes have to be machined for a flange face o-ring seal. Here is a link to a bootleg copy: not sure what version. http://dozerparts.ru/f/tsilindry_i_uplotneniya.pdf
In a quick look I see where the gland has oversize seals available but they have them for the piston seals as well where presumably the cylinder could be honed up to .060" oversize? I've never heard of oversize seal kits for cylinders. O-rings and such sure but not all the cylinder seals. Anytime I've needed cylinder seals there's only been one part number for the seal kit. .060" could take out some pretty deep grooves.
Honing a scratched cylinder is a great way to save a few bucks and salvage a expensive component. You can also purchase Cat Reman oversized bare cylinders vs purchasing new. I spent a long 18 long month rotation in a Cat dealership hyd spec shop rebuilding cylinders. Holy cow, that got old after about two months.
Some really good information in this thread Thanks for the link to the hyd book
I would guess oversize cylinders/seals would only be available for larger machines and not skid steers or TLB's.?
That would be my assumption as well, Beuhler?
If you still think its in the cylinders, (bypassing internally) raise your boom up safely hold the boom up in the air. Release your hydraulic pressure from the machine( key on no start...rotate and move joysticks) to release pressure. Remove your lift cylinders hyd hoses ( both) and cap both of them off with metal caps. Take a permanent marker and mark your rod. Then releases the boom from whatever you where holding it up with. Watch to see if the boom drops down. If it does its in your cylinders...if not you know its bypassing some where else. Possible control valve spool...etc. good luck... let us know what you find.
Has this machine got an emergency loader arm lowering valve? I've seen them malfunction and cause a creep-down condition.
Not to be argumentative, but capping both lines will create a displacement cylinder, still won't tell you if the pistons are leaking by? you can also just raise the bucket up a little bit prop it up with a short wood block, disconnect one rod end hose at a safe place and put the hose end in a catch can. With an appropriate tool knock the block out from under the bucket. If one piston or both are bypassing you will get oil coming out of the rod end port(s) Safety first though.
The thing with standard hydraulic cylinders is if the piston seals are gone, they will only drift out, not in.
Depends on the rest of the system and the load imposed. When they go into displacement mode, with only the area of the rod to hold the load, pressures can shoot up increasing the typically minor leakage in the valves, or in some cases opening the port reliefs.
Definitely. I meant just a sole, isolated cylinder should not drift inwards.
I took apart the lift cylinders again to take measurements with calipers and bore gauge. piston rod shaft diameter = 40mm (39.96mm) piston head seal diameter = 60.55mm (largest diameter seal on the piston head) piston head wear rings = 59.90mm cylinder bore = 60mm + 1/1000inch there isn't much force required to pull the piston out of the bore. I can pull the piston out with one hand, and push it back into the cylinder with one hand (50lbs of force required maybe) These clearances are probably too loose, and is making the lift cylinder operate in displacement mode?
It doesn't seem like the seals are the problem in my opinion.
Was the boom lowering valve checked? Were the boom UP and Down relief cartridges swapped?
I'm lurking
This would work if the lift cylinders dropped without any weight on the bucket, and it seems its only dropping with a load. but yes it would work, I know cause I did this with a backhoe, but the lift cylinders where dropping without a load, turns out when capped the boom did not drop. spent more time troubleshooting and turns out the spool in the lift valve was damaged with a big deep scratch. I guess in this situation if the same was applied, but with a load on the bucket youll get a good test to verify if it is in fact the cylinders or not. I havent seen the machine but I dont think it is the cylinders. Its something else.