Three points hitch: mast adjustement on ASAE S217.12

[pietro82]

Hi all,

I'm designing a three points hitch and I know there is the ASAE S217.12 that imposes some geometric constraints. I don't understand one constraint called "mast adjustment height", it isn't drawn on the standard. Does anyone know, what it is?

Thanks

 

[pietro82]

Thanks for your reply. Regarding to the three point hitch design, I have found this paper,

https://kb.osu.edu/dspace/bitstream...neeringinAgriculture_2007_23_1.pdf?sequence=1

where a CAD tool has been created to help the designer to design a three point hitch able to follow the ASAE S217.12 standard. In the three point hitch design I think it is also necessary to take into account the lifting perfomance measured in according of the OECD code. There I created a tool to design a TPH. Would it be useful for designers to take into account to design a three point hitch?

 

[pietro82]

I have another doubt regarding the three point hitch. I'm looking to nebraska test report of some tractors and I noticed the three point hitch can lift a load higher than the tractor stability limit. (As example check the CASE IH Puma 155). This sounds weird to me, since it's not a safe working condition. Why are three point hitch designed to develop so high lifting loads?

thanks

Pietro

 

[IceStationZebra]

I have seen the test done with a bare tractor frame with no engine or tires bolted to the floor. There are no stability limits in the testing so the tractor manufactures keep pushing the limit so they can claim to have the strongest tractor. Which, like you pointed out can get silly.

In reality you don't want a lack of lifting force to be lowest limit in your system. This will make customers think you have a weak tractor. Farmers will add weight to the front of the tractor to offset the lack of stability. (water/salt mix in tires, iron weights) I have seen large water tanks mounted to the front just to add ballast. On the negative side this is stressful to the tractor structure, adds to soil compaction, and burns extra fuel.

ISZ

 

[pietro82]

I noticed all companies are set up to a similar lifting force for the same tractor type. I was wondering why just that lifting force was choiced by producers. I think somehow it's the force necessary to lift the implement at a specific vertical acceleration. Do you think it's a good argument?

 

[IceStationZebra]

Many tractors have the ability to adjust the lift/lower velocity, especially with automatic draft control. Of course the acceleration rate comes into play with this control loop, but I can't recall ever observing any significant acceleration/deceleration control ramp. They are usually fairly abrupt.

I'm not suprised that similar tractors from different manufacturers have similar lifting forces. As customers compare specifications nobody wants to appear lacking. There are always the 1% of customers that will push the limit, but like you suggested previously there is a practicle limit that most customers will not exceed.

ISZ

 

[pietro82]

Thanks for your reply. I got the point. So when designers design a three point hitch, what do they take into account?

 

[IceStationZebra]

Being cynical, I would say the competition! ISZ

 

[pietro82]

mmm but how can they beat the competition?

By the way...I found an old paper, where it's written it is necessary do apply two times the gravity force of an implement to remove it out from the ground. Taking into account the heaviest implement , I get the same order of size of the maximum lifting force for a specific tractor size

 

[pietro82]

Hi IceStationZebra,

I have just realized that in a normal implement the CoG is moved rearward than the one used at the OECD test. Therefore in normal working condition the lifting force is lower than the one measured with the OECD test. What do you think about it?

thanks

 

[IceStationZebra]

One has to realize that these standards are created by representatives from the companies who are selling these products, not independant engineers or scientists. And as such they all have an interest in making their products look good. They could have chosen a distance that was farther back, but then the number would be smaller and infer less strength. Sometimes the standards come from a group that already has a standardized process, maybe has done some comparative testing, and the others just agree to adopt it. In this case it may have come from the Nebraska tractor test, but I don't really know.

I can tell you from experience that some things are very black and white while other aspects are not. One example is taking production varience into account. If you did a statistical analysis of your 3pt hitch design there is probably a +/-0.5% possible varience. Though the difference is small, what number gets put into the sales literature?

ISZ

 

[pietro82]

IceStatioZebra, thanks for your reply, you're right, also to understand there no information to understand why the ISO or the OECD choosed a specific test set-up. About the variance no number is put in the sales literature...