View Full Version : max rpm
to you guys who have experience with setting rpm.
i was adjusting my 20 command today under blade load.
i really left her humming.what would you fellas think the max rpm should be both for performance and longivity. any info appreciated.
06-26-2001, 07:22 PM
Check the operators manual for high rpm setting. It should be around 3600 rpm with NO LOAD! If you set it under load you will damage the engine at full throttle with no load, the governor will adjust rpm for load. If you are needing more power, I suggest buying a large engine.
THANKS FOR THE INPUT CLAY. GUESS I BETTER TAKE IT UP AN HAVE IT CHECKED.THE ENGINE IS PLENTY STRONG FOR LAZER 52. JUST WANT IT AT PEAK SO AS TO TAKE ADVANTAGE OF MACHINE POTENTIAL. LATER
A 20 Command will have no problems with 3750 RPM's NO LOAD. You never adjust RPM's under load. With blades engaged this should bring you down to 3650 or so.
THANKS DIXIE ,I THINK THATS ABOUT WHERE IM AT NOW.
I KNOW SHES REALLY PERKING BUT STILL SMOOTH SO THINK IM ALRIGHT.
06-27-2001, 05:37 AM
You should put a tach on the engine to see exactly how many RPMs the engine is turning. It is impossible to guess how many it is turning.
THATS THE PLAN. im going to check it out of curiosity if nothing else.ill post if it turns out ive got it at 3900 or something like that.later
06-27-2001, 11:46 PM
I just read my Command owners manual,all command V-twins can run at 3750-no load as the max rpm's.This is right from Kohlers book,so thats where I d keep it.
Vandora Lawn & Landscape
06-27-2001, 11:48 PM
Where do you get a tachhometer for mower engines? Will the same one work on trimmer engines?
By trimmer engines, what do you mean? Push mowers, string trimmers? Most all small engines will spark every revolution which allows for these inductive pickup tachometers to work on all of these. In my opinion, every mower should come from the factory with a digital tach. Oh and you can find them at your dealership or any of the popular mail order parts catalogs.
06-27-2001, 11:56 PM
Tiny Tach makes one for one or two cylinder motors. My mower came with this on it, but dealers carry them also. This is a combo Tach and Hour meter and some are resetable, don't get the resetable one if you have kids. They like to push buttons and then it is reset. :(
Vandora Lawn & Landscape
06-27-2001, 11:59 PM
06-28-2001, 12:16 AM
lanscapersupply.com or jthomas catalog carry tiny tachs for 39.95 or so. they work great for tuning trimmers for max rpms back when you could actually TURN screws on trimmers. Dave g
06-28-2001, 04:03 AM
Push mowers, string trimmers? Most all small engines will spark every revolution which allows for these inductive pickup tachometers to work on all of these.
A 2 cycle engine sparks on every stroke and a 4 cycle engine sparks on every other stroke. The inductive pickup tachs like the Kohler and Tiny Tach will only read correct readings on a 4 cycle engine. You need to double the reading for a 2 cycle engine.
06-28-2001, 10:16 AM
A good hand held tach is a very valuable piece of equipment in this business. If you have Employees it's even more valuable?
I bought a Solar - Battery powered one over 15 years ago, it does 2 & 4 Cycle & up to 8 cylinders. It was $150 I think & has Saved me Plenty of bucks in the years & I use it quite a bit til this day.
Every once in a while an Employee will get a Brain Cramp & Jack up the RPM's. This shortens the life of your engine quick!
I was told by a guy that had worked for B&S for many Many years that every 10% you raise the Engines RPM's over the Manufactures Suggested Maxium you shorten the life of your Engine by 20%.
No load & under load anything less than 150 RPM's change is acceptable if much more drop in RPM's you probably need to adjust the Govenor, if the Engine is running at Maxium Manufacture Suggested RPM's.
06-28-2001, 12:42 PM
You shouldn't be so worried about what max rpm your engine can perform at. You should be more worried about the blade tip speed. The industry standard is 19000 ft/min. If this is altered after the machine has been set up by either the manufacturer or a dealer the manufacturer is no longer liable for potential failure or bodily harm to others. Your running a buisness so this is much more critical than a homeowner who alters his machine. If you post your blade size and pulley sizes on both your blade spindle and engine I can calculate what rpm is safe for your machine to run at. Hope this helps.
MOWINGMACHINE, i assume motor rpm will directly relate to bts.
my machine is 52 with 18 inch blades so im probably not as fast
on the blade as the wider decks.any info you come up with will be appreciated.thanks and later
06-29-2001, 12:01 AM
There is a direct relationship between the engine rpm and the bts. I really need to know what size pulleys your engine has and also your blade spindles. If they are the same size then the gear ratio is 1:1 so the engine rpm is the same as the blade spindle. Without these pulley sizes the rpm could vary drastically. Hope this helps.
AWM has nothing to worry about in regards to exceeding max. allowable blade tip speed. He does not have a prototype home built lawnmower, he has a well engineered Exmark Lazer. His adjusting his RPM's from 3600 to 3750 will have no adverse affect on the safety of the mower. I have personally heard of some mowers cranked up to 4000 RPM (Kohler Command) with no adverse problems. NO, the blades will not split apart or come dislodged from the spindle just due to blade tip speed. Out of balance.....MAYBE.
AWM, let us know what the RPM's were set at from the factory.
hey dixie ,now im really confused,not worried cause the engine is
running the way i likum to, but reading the manual it says
max torque 32(44)@2500 . which sound like a lot less rpm than
i would have thought.Mabe that means at 2500 is where it reaches
max ft lb torque. ive listened to engines most of my life an this thing sounds like she is over 3600.well if i ever get up there ill have it checked. i guess i mentioned that i have the kohler command 20. mowingman there was a 1 inch larger pulley at the blade drive
than at the motor.larger hard to get to but appeared to be 7.5inchs.
well the grass has been dry enough for a while now so got to get.
appreciate everyones input. laterTM
06-29-2001, 12:49 PM
I agree with you for the most part. A well engineered product most likely will not have any problem when exceeding the blade tip speed standard. The safety standards are what well engineered products are designed around though. Notice in my post I also spoke about a much worse safety issue and that is bodily injury. If a thrown object were to leave your mower and hit somebody and the situation was serious enough you better believe there will be a lawsuit. Then there is going to be an investigation. When they determine your mower was exceeding safety standards it isn't the manufacturer that will take fault for the accident. It is you the one who altered the original product that will take full responsibility. Safety standards are set in place for a reason and until you get an engineering degree and can prove the standards to be unreasonable I suggest you follow them.
are you sure your blade pulleys are larger than your engine pulley it should be the other way around for an 18" blade. Otherwise the blade tip speed would be really slow considering the max rpm on your engine.
06-29-2001, 02:16 PM
2500 is where the engine makes maximum torque. The torque begins to fall off after 2500 RPMs. The horsepower usually continues to increase right up to the maximum RPMs and beyond. The RPM ceiling is a compromise between engine longevity and having enough of a pad between max RPMs and maximum torque.
Look at it this way...
If you were to run the engine (not just on a mower but in any application) at maximum torque (2500 in your case) and you were to encounter a force which will drag the RPMs below maximum torque then the engine would possibly not have enough power to regain RPMs before stalling and the lower the RPMs go the less torque the engine will have available to regain RPMs.
Now if you take the exact same scerario and raise the RPMs to 3600 then as the force is applied and RPMs drop the engine is actually making more torque and can more easily regain RPMs.
If you were to take your mower and run the RPMs at 2500 you would need a more powerful engine to cut grass.
i see i see said the blind man. just kidding ,i was talkin to a mechanic today(WHICH BY THE WAY YOU SOUND LIKE YOU ARE) and he told me the same thing you did.in any case shes running good. i got my modeck yesterday so im eager to try it out. later and thanks.
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