how to build your own custom backhoe subframe

[dfkrug]

Not many tractor owners are willing to go thru the process of making a
custom subframe to mount a backhoe attachment securely to their tractor.
I want to encourage anyonewho is reasonably handy to consider doing
this....it is very rewarding, and you can endup with a TLB setup that
comes close to what you would get if you had a factory-integrated
Tractor-Loader-Backhoe. A custom subframe can be built BETTER than an
add-on subframe from a maker of hoe attachments. More on that later.

I get asked to build a subframe for someone else's tractor now and then. I
never take them up on the offer, but I am happy to show them how I do it.
I have helped some folks long distance, but that is difficult. I want to use
this thread as a go-to place for those considering the fabrication of their
own subframe. In my approach to design and fabrication, I always want to
have both the hoe and the tractor here: measure 3 or 5 times, and cut only
once, hopefully. I have learned a lot about backhoe attachments and
subframes by owning and making them, and by analyzing the many samples
I have seen. I have a large database of hoe attachment specs, but I
am always looking for more data and new ideas.

In this thread, I will show the process I went through to design and fabricate
a subframe to attach a Prairie Dog 7.5 backhoe attachment to a John Deere
4300 tractor. I will use this and other examples of subframes to illustrate
different design approaches. Comments and questions are welcome.

But first, I want to discuss the basics.

What is a subframe? It is a structural member(s) that is rigidly tied
with welds, bolts, or pins to the main structure of the tractor. The
subframe rigidly transmits the forces from the implement to multiple parts
of the tractor's structure. By distributing the forces caused by the
implement, the tractor is protected from excessive concentrated forces, including excessive tension as well as destructive bending moments.

Subframes are used as a superior way to mount a backhoe, instead of
using the Three Point Hitch (TPH). There are many cases where a
Category 1 TPH has been used to successfully mount a backhoe.
There are also numerous documented cases where TPH-mounted hoes
have broken the toplink mounts, or cracked tractors in half. That is
no news to TBN members. Subframe-mounted hoes can also provide a
better integrated tractor with the hoe mounted much closer in to the
rear of the machine. A closer-mounted hoe will increase the minimum
departure angle, which is a fancy way of saying it won't hit the
ground as often when driving through a depression, or onto a trailer. Some
off-the-shelf subframes do not mount the hoe any closer than a TPH mount,
however. A closer-in hoe mount will also improve a tractor's drivability.
If a large weight it mounted far from the center of gravity of a tractor,
it will have what is called a high polar moment of inertia. This inertia
will resist changes in steering input.

In the first photo, I show the parts that comprise a 3PH hoe attachment
mechanism. On the left are the parts that are permanently bolted to the hoe.
The other parts comprise a much stiffer toplink, and brackets to keep the
lower 3PH arms (not shown) from rising. The second and third photos show
the same tractor on the 3PH, and on a Woods subframe, respectively. This
early Woods subframe does not mount the hoe any closer to the tractor
than the 3PH mount does.

Next: subframe types

 

[Robert_Parent]

Very interesting.. I look forward to the next chapter..

I would really like a small backhoe for my Kubota BX and have been giving much thought to building or modifying pieces of a purchased stand alone system.

Thanks,
Robert

 

[mmurphy]

I am going to watch with great interest. I have been thinking of ways to set up for my 416 Allis G.T... My first part will probably be a one arm loader, and then maybe a backhoe.

 

[D.James]

Very good info. I look forward to the next post.

Later
D.james

 

[magicheater]

I had the very same Prairie Dog Digger. A very quick and capable hoe with the pump unit they have. Sold it in pursuit of a larger unit.

 

[David Devuono]

I plan to add a subframe for my backhoe as well. I don't so much have a problem when digging as when I want to use the hoe to move the tractor around a bit. Thats when things get iffy and I wish I had a sub frame. thanks.

 

[dfkrug]

There are two major types of subframe used to mount backhoes to tractors:
The first I will call a ladder frame because it usually has two long beams
that go underneath the tractor (first drawing), connecting at the rear axles
or differential housing, and somewhere forward of the tractor's center. Some
form of toplink is usually required, too. My crude drawing is a side-view
schematic. Sometimes there are "rungs" on this type of subframe for more
structural stability.

The second major type of subframe avoids the use of the tractor's 3-pt
toplink in favor of four pinned or bolted attachment points. This requires
that the subframe wrap around the rear axle housings, so it must be
detachable in that area. These "Four Point" subframes are what you
will find on most modern CUTs and purpose-built TLBs. As a bonus, this
type of subframe is often very quick mounted/dismounted. This leaves half
or more of the subframe mounted to the tractor, even when the hoe is
removed. See the second drawing.

These 2 types could just as easily be distinguished at the "undermount"
type, and the "axle-surrounding" type. Some subframes are the latter
style, but don't use four detachable points. The point is the two categories
are not perfect.

 

[dfkrug]

There are some rather odd subframes that don't fit either of these
categories. I have seen subframes from Deere for both the 955
and the 4300/4310 that are little more than small brackets that bolt
to the sides of the rear axle housings (photo). This approach has not
worked well, however, as JD later required users of their hoes to buy a
separate reinforcement subframe to add to that. I have posted a page
from JD's Subframe Installation Manual, which is dated 2005. That could
mean that this reinforcement was developed in response to failures
in the field. Like the one in the below photo. Here is a TBN thread
I posted about one of these broken JDs:
http://www.tractorbynet.com/forums/john-deere-owning-operating/146426-jd-4300-broken-half.html

The two small brackets are mounted using the same bolts that hold the
aluminum axle housings to the cast steel center gearcase. This has also
been a reported problem area. JD strongly advises users of backhoe
attachments on the JD4300/4310 to use the additional reinforcement.

This has not been much of an issue with the 955, since that tractor comes
with a full external frame, unlike most CUTs.

 

[jdmar]

I trust your knowledge and skills from your other posts...now you are scarring me! I have a 4300 with the 47 hoe on it. My tractor is not currently cracked in half and I would not like that situation to change. Do you know what part/kit# there is to add that subframe support to the tractor? Looks like it is smart insurance to me. I plan on keeping my tractor a long time. One thing I liked about this Deere was the integral backhoe support that seemed to me (oops) to be stranger than many of the 3PTH models out there. I liked that you lifted with the 3PTH but then the actual weight was carried by the steel plates bolted to the tractor. Looks like this is why I am not a licensed engineer!

Peter

 

[arrabil]

Thanks dfkrug.

 

[dfkrug]

Do you know what part/kit# there is to add that subframe support to the tractor?

The JD document I took the snip from is titled "Product services Information"
and has part # LVU17203 at the bottom. That part # does not come up on
JDParts when I enter it there.....maybe it is "whole goods", and not "parts".

I think I first heard of this doc on TBN....maybe someone else has tracked
this down?

Anyway, I will next show how Woods and Bradco make subframes for the
JD 4x00/4x10 tractor, and my own solution.

 

[Baby Grand]

Keep it coming - this is good stuff!

 

[dfkrug]

Here is a factory subframe from Woods for the JD4310. The photo shows
it mounted to another Prairie Dog, and it is of the ladder style. You can
see the enhanced toplink if you look closely.

Woods provides subframes for both the 4x00 and the 4x10 tractors, and
they have different part #s, but the installation manuals show identical
drawings (2nd photo). You can see that they utilize the 3PH lower arm
pivots as the only attachment at the rear. I think this is not a good idea
because the 3PH pivots are part of an all-aluminum axle housing. Followers
of my 4300 rebuild project have seen that that aluminum casting is not very
strong (3rd photo). BTW, Woods does not list subframes for the JD 3x20s,
which use even more aluminum back there. Woods manuals for all their
subframes can be viewed in PDF on their website.

 

[arrabil]

What do you think about a Woods-type subframe that ties the lower 3pt mounts to above the drawbar? The drawbar is capable of supporting 1000lbs on the 42/43/44x0 and so it would distribute the strain on the lower 3pt mounts and prevent the subframe from twisting them off too. [This is what I was considering as a starting point.]

 

[dfkrug]

Bradco produces a subframe for the JD4300/4400/4310/4410 that is a bit of
a hybrid of the two types. It has four mounting points, but it does not
wrap around the top of the axle housings. They avoided hanging anything
off the axle housing (good!), and instead uses the ROPS bracket for rear
support. The ROPS brackets are 1/2" plate steel, secured with four 19mm
grade 10.9 (similar to SAE grade 8) bolts. Bradco replaces the bracket with
their own, with added weldments. This is superior to what Woods has done,
and I will do it this way, too. The pix are drawings from the Bradco
installation manual.

The subframe does not fully wrap around the axle because there is simply
no room! This was a challenge for me, since I wanted to make a 4-pt
style subframe that wrapped around the axle and did not depend on a
toplink. Bradco eschewed the toplink, but I don't feel comfortable with
the potential flex in the area I marked on the drawing. I am going with
an enhanced toplink. See the first drawing yellow arrow annotation. To
give Bradco credit, they probably build that part of the subframe very
stout to avoid this problem. I think the toplink will brace against these
forces successfully without the additional strengthening necessary.

Because the front mount for the subframe is much narrower than the
ROPS brackets, Bradco put some Z-bends in the frame rails (#3 in the
2nd drawing). I used this approach for one of my 955 subframes. It is easy
to fabricate, but you will need to add a brace (#5) if you do it this way.
Bending forces on a frame element with these bends will cause twisting
if no brace is used.

To reduce the wiggle you get with pins, Bradco uses wedges (#1) to secure
the top 2 attachment points. This is an interesting approach that I find
intriguing, and I have a lot of respect for Bradco engineering. I have no
personal experience with this wedge approach and how it works to limit
wiggle in the field. I do think wedges with rectangular cross-sections and
the holes for them will be more laborious to fabricate. If others have
experiences with the wedge approach, I hope they comment here.

 

[Johndeere3720]

Keep it coming - this is good stuff!

This is really good stuff!

 

[dfkrug]

What do you think about a Woods-type subframe that ties the lower 3pt mounts to above the drawbar? The drawbar is capable of supporting 1000lbs on the 42/43/44x0 and so it would distribute the strain on the lower 3pt mounts and prevent the subframe from twisting them off too. [This is what I was considering as a starting point.]

There are 4 bolt holes in the bottom of the gearcase steel casting...these
are used for the drawbar brackets. You could take the drawbar off and
use the holes to reinforce the subframe, but you will lose the Woods
quick-attach capability. The QA capability is too often used with the
sacrifice of structural integrity.

Note that it is better to support the hoe with connections that are as
far outboard as possible. This will reduce bending stresses on the axle
housing, and those long bolts that attach them to the center gearbox.

I will be posting some underside pix of my completed subframe shortly.

 

[jdmar]

Thanks for all the info. I am now only digging 6" deep in loose sand!
Peter

 

[dfkrug]

Woods makes hundreds of different subframes for many many tractors.
This is one of the reasons that the makers of the Prairie Dog hoe copied
the structure of the Woods 7500. The earlier subframes sold by Woods
were of the ladder type, and they often required you to mount brackets
to the tractor carcass that blocked off the 3-pt hitch lower lift arms. This
made it very tough to use the 3PH when the hoe was dismounted. The
ladder frames also extended very far to the rear; I suspect that was to
provide clearance for the seat. Woods did not optimize their subframes
for the closest possible hoe mount. Some of Woods' subframes for
different tractors are so similar that I suspect that they also tried to
minimize how different each one was to limit manufacturing costs. Kind
of like a "least common denominator" approach to design. A custom subframe
can improve that by optimizing for a particular tractor.

As a point of reference, an integrated TLB will have a departure angle of
as much as 20 degrees; an attachment subframe is rarely better than 15
degrees.

I have purchased and installed only one Woods factory subframe (on the
L2550 above), and I found the price to be reasonable (about $500), but
the wait was very long. Their newer 4-pt subframes are over $1000.

The pix show Woods 4-pt subframes for Kubota L-series and B-series. In
the latter case, you can see that the subframe does not have a structural
member that wraps around the rear axles. Lack of clearance is the likely
reason. I like how robust these units are, and their use of existing threaded
holes in the Kubota castings where possible. (Note that Kubota and
Kioti have many unused threaded holes available on their tractors, while JD
has few. I don't know about other tractor brands.)

It is also interesting how Woods' design strategies have evolved. Newer
subframes no longer block the 3PH, and earlier pinned connections are
often replaced with secured bolts. They even provide a wrench to help
speed up the mount/dismount process (3rd photo). These last 2 photos
and the first one show Kubota Grand L 4-pt subrames.

Pin attachments are quick to install or remove, but they wiggle. Bolts are
more secure, but they can loosen up and they take longer to attach
and remove. Any slop in rigid mounts will allow wear in the holes. Bradco
attempts to find the best of both worlds, with pins and wedges.

 

[magicheater]

Here is what I have done so far to modify a sub-frame not made for my tractor.Not a lot to do but did have to work within some limiting factors. Had to drill an extra hole in the forward brackets. Had to widen the the rear portion of the framework to match up to my tractors dimensions. Couldn't go too wide or I would have to modify the hoe mounts also. The forward to aft dimensions were spot on, otherwise I would not have purchased it.