Hour meter (HOBBS) vs actual run time.

[HiPowerOne]

I had noticed before, but recently I really took a look at the hour meter during operation compared to actual clock time. I have been running my tractor at about 1700 rpm for several hours and compared the recorded hour meter readings to the actual clock time. 1700 rpm is about 60% of 2900 full rpm. Then I looked at the recorded hour meter time and it was about 60% of the actual clock time. Has anyone else noticed this?

 

[old and tired]

So, your hour meter is working perfectly, as it should.

Yea, that's how it works....

 

[GreensvilleJay]

Wonder what committee decided to that 'calibration'.
Hmm, does the time increase at lower RPMs, say below 1200 ? As well all know running a diesel at low RPMs is not good on the engine,so service needs to be done sooner.

 

[woodman55]

If your tack/hour meter has a drive cable, what it is actually measuring is the total crank revolutions, just like the odometer on your car. The manufacture then just takes a guess at what the average rpms will be, does some math to find the # of revolutions in a hour, and uses that as one hour of usage. Some of the old tack/hr meters would state, hrs at xxxx rpm.

 

[Henro]

If your tack/hour meter has a drive cable, what it is actually measuring is the total crank revolutions, just like the odometer on your car. The manufacture then just takes a guess at what the average rpms will be, does some math to find the # of revolutions in a hour, and uses that as one hour of usage. Some of the old tack/hr meters would state, hrs at xxxx rpm.

View attachment 134661

Maybe I’m the only one, but I never had a vehicle where the odometer measured engine crack revolutions…

I have a tractor with a cable driven our meter and another with an electrically driven meter. The electric one is just like a clock. It increases one hour when my watch increases one hour.

The cable driven hour meter is designed to change one hour for each hour the engine runs at PTO speed generally, but it could be defined as any speed. So if PTO speed on my tractor is 2500 RPM to get 540 out of the PTO, then if I run the tractor at 2500 RPM for one hour, the hour meter will increment one hour. But if I run the tractor for one hour at 1250 RPM, then it will take two hours on my watch before one hour increments on the cable driven our meter.

Woodman55, is this what you were saying? Or were you saying something different?

 

[GreensvilleJay]

re: I never had a vehicle where the odometer measured engine crack revolutions…

well, if the odometer is mechanical , 'technically' it does....
turning engine-->clutch-->tranny__> speedo gear-->cable-->odometer

there can be a HUGE difference in engine 'condition' between odometer vs hours readings.

take a fire truck, low mileage but tons of hours (idling at scene, pumping water )
cop cars, idling for 9 HOURS(gotta have A/C on..) waiting for bad guy to come out !

 

[woodman55]

The odometer was just a reference. Greenville explained well.

 

[woodman55]

Maybe I’m the only one, but I never had a vehicle where the odometer measured engine crack revolutions…

I have a tractor with a cable driven our meter and another with an electrically driven meter. The electric one is just like a clock. It increases one hour when my watch increases one hour.

The cable driven hour meter is designed to change one hour for each hour the engine runs at PTO speed generally, but it could be defined as any speed. So if PTO speed on my tractor is 2500 RPM to get 540 out of the PTO, then if I run the tractor at 2500 RPM for one hour, the hour meter will increment one hour. But if I run the tractor for one hour at 1250 RPM, then it will take two hours on my watch before one hour increments on the cable driven our meter.

Woodman55, is this what you were saying? Or were you saying something different?

Yes. That is what I was trying to get at.

 

[Henro]

re: I never had a vehicle where the odometer measured engine crack revolutions…

well, if the odometer is mechanical , 'technically' it does....
turning engine-->clutch-->tranny__> speedo gear-->cable-->odometer

there can be a HUGE difference in engine 'condition' between odometer vs hours readings.

take a fire truck, low mileage but tons of hours (idling at scene, pumping water )
cop cars, idling for 9 HOURS(gotta have A/C on..) waiting for bad guy to come out !

LOL… You forgot the account for the slip in an automatic transmission! Any slip introduces error so the measurement is an estimate rather than an exact calculation. On top of that, you get different speeds for a given RPM dependent on what gear you’re in. So the speed doesn’t represent engine, RPM in every case.

Nice try though…

Edit: I just noticed the typo in what I said that you quoted. I must concede that the odometer may very well measure crack revolutions. At least if I correctly understand what the slang term crack means…

 

[GreensvilleJay]

aw, don't get 'cranky' !
It's easy to have a 50 cent computer calculate actual AT slip and know what gear it's in, then display an accurate speed.

 

[Henro]

aw, don't get 'cranky' !
It's easy to have a 50 cent computer calculate actual AT slip and know what gear it's in, then display an accurate speed.

I suppose so. But do they account for different gears that are selected? Of course they could. Now how many tractors have these computers on board? Case closed. LOL

 

[GreensvilleJay]

I didn't say they do it, just that it CAN be done.
I've spent 5 decades doing 'proof of concept' design prototyping including getting hoppers full of fertilizer on log skidders to evenly distribute the material regardless of ground speed, engine RPM, tranny gear,etc. NB has some real 'interesting' hills in the bush !

 

[Henro]

I didn't say they do it, just that it CAN be done.
I've spent 5 decades doing 'proof of concept' design prototyping including getting hoppers full of fertilizer on log skidders to evenly distribute the material regardless of ground speed, engine RPM, tranny gear,etc. NB has some real 'interesting' hills in the bush !

Just about anything can be done. Since you were replying to the question posted, I figured your reply was intended to be an answer to the OP’s question. My mistake….

 

[PoTreeBoy]

aw, don't get 'cranky' !
It's easy to have a 50 cent computer calculate actual AT slip and know what gear it's in, then display an accurate speed.

Last i checked, it was easier to measure the transmission output speed/revs. Then all you have to adjust for is axle ratio and tire size.

 

[DustyRusty]

Just about anything can be done. Since you were replying to the question posted, I figured your reply was intended to be an answer to the OP’s question. My mistake….

Were you on the debate team in high school?