# Slip clutch for post hole digger (PHD)



## JGPenfield (Mar 10, 2016)

I break bolts all the time with my PHD and I wanted to switch my shear bolt to a slip clutch. I have the slip clutch with smooth ends and I was all ready to install and then cut the shaft for the extra length. But I read a few posts that suggested that PHDs should use a shear bolt and not a slip clutch. On top of that I could not find PHDs that came with a slip clutch. 
1. Is there a reason not to use a slip clutch on a PHD?
2. Does anyone know of a manufacturer that makes a PHD with a slip clutch from the factory?
3. Is there anyone out there using a slip clutch on a PHD? If so have you had any problems with it?


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## RC Wells (Dec 26, 2008)

King Cutter has slip clutch post hole diggers. http://www.kingkutter.com/store-product.aspx?id=2147483701

Cost is the primary reason most three point diggers come with shear bolt drives.

If you convert to a slip clutch you need to make sure the release is set to the drive torque specifications of the digger to avoid damage to the equipment.


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## pogobill (Jan 31, 2012)

I was wondering how long you've had the digger? And how big of an auger you are using? I have rock strewn clay around my place, and a lot of tree roots. If you take your time and be patient, use a big honkin pry bar when you start clipping big boulders and roots, you can minimize the frequency of those dang bolts shearing. It took me a while to sort it out, but I don't shear the bolts very often anymore. I admit that I can take a long time to bore a hole at times, but it's not because I'm broke down. While on the subject, I'm thinking of getting a 6" auger to see if I'm more successful than with the 9" one.


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## Hoodoo Valley (Nov 14, 2006)

Mine is a 12 inch and I have essentially river bed at my place, so lots of shear bolt change outs.


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## JGPenfield (Mar 10, 2016)

My digger is about 2 years old. I am probably using too small of shear bolts. Since I already have the slip clutch, I am going to try it. I will check specs of PHD and the slip clutch and make sure they match. 


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## JGPenfield (Mar 10, 2016)

My PHD is a county line S240455TSC made by Speeco. It uses a 3 inch grade 5 , 5/16 inch shear bolt. It does say grade 5, not grade 2 in the owners manual. 
I have a GR-1 Slip clutch #90653 from Agri supply. From the factory it should slip at 9293 inch pounds and is made for series4,5 or 6 PTO shaft. It does not give values for adjustment like other clutches. Would this work on my PHD or will it require too much torque to slip?


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## RC Wells (Dec 26, 2008)

You ask a very complex question that is not easily answered, but I will try to give you some tools to consider in developing what answers you can devolve from available information.

I think every farmer has been down this road at some time or another, so you are not the lone wolf with this type of question.

.6 or 60% of the tensile strength of the grade 5 bolt is the single plane shear strength, the PTO drive uses a double shear. So material thickness of the shaft and yoke become factors to include in the calculations, along with a safety factor that is fairly standard in today's litigious society.

Here is a reasonably accurate chart of typical bolt strength:
https://www.almabolt.com/pages/catalog/bolts/proofloadtensile.htm

Here is a reasonably good calculator of double shear strength:
https://www.engineersedge.com/material_science/bolt_double_shear_calcs.htm

You will then need to convert from foot pounds to inch pounds, and here is a calculator for that:
https://www.convertunits.com/from/inch+lbs/to/foot+lbs

I cannot add anything of value to the consideration of capacity adjustment of the slip clutch, as that calculation would require friction coefficients of all friction materials used, and the loading capabilities of the bolts and springs if you wish to vary from the stated capacity. I have not seen manufacturers provide that information on a consumer product in over 20 years, because they have liability issues regarding intended use.


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## RC Wells (Dec 26, 2008)

Here is a quick and dirty alternative to determining torque loads. A bit of "Kentucky Windage," but will get you in the ballpark of the torque load values you need in order to determine suitability:

Implements and tractors have torque ratings determined as instantaneous, also known as the horsepower second value.

Horsepower Second Value:
There are 550 foot pounds to a horsepower.

Take the manufacturer's horsepower rating for the implement, multiply be 550, and that is the maximum foot pounds of torque for which it is designed.

Take the total foot pounds value, run it against the conversion algorithm in the above post to determine inch pounds, and you will have your answer expressed in a value you can compare to your slip clutch rating.

Or, take the 9,293 inch pound rating of the slip clutch, convert to 774.4166663569 foot pounds, then convert that value to horsepower. Divide the number of foot pounds, 774.4166663569, by 0.00181818 to convert to horsepower. My quick and dirty result is roughly 1 and a half horsepower limit for the slip clutch.

Alternatively, if the post hole digger were rated at 20 hp, you would divide 20 by 0.00181818 to get 11,000 foot-pounds per second. This converts to 132,000.0000528 inch pounds of clutch load requirement to operate the digger.


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