Offbeat
Engineering
Case Study: Bale Cutter
Problem Definition
Sometime in 1999, we were asked by Scottish hay producer Francis
Peto to investigate the possibility of building a machine which could
cut 1.2m x 1.2m round bales into neat slices for use by his "mainly equine"
clients. He had already tried several methods of cutting including using a
chainsaw, which had proved wholly unsatisfactory, and somewhat
dangerous!
After discussing various approaches with Francis, we finally settled
on a machine that should meet the following overall requirements:

•  It should be capable of cutting bales into "slices" as narrow
as 300mm wide. Whilst still retaining the bales integrity.
•  It should be fully mounted on a tractor to allow on the move
cutting.
•  It should have the facility to pick up and deposit the bales.
•  It should be driven by the tractors hydraulic services allowing
the cutting cycle to be automated.
Design Considerations - An Iterative approach
As one might imagine, for a job of this nature, it was not possible at this stage to
quantify some of the parameters involved in cutting a bale. For example, major
unknowns included the following:
•  What type of blade tooth profile would work best?
•  How much stroke should the blade have?
•  What would be the force required to reciprocate blade?
•  What speed could the blade be reciprocated at?
•  How much force would be required to drive the blade
down through the bale?
It was clear that because of these unknowns, the design would have to be flexible
enough so that these parameters could be changed without major rework or
redesign.
Design and Build - From CAD to Metal
Our approach was to develop a custom computer program, which would animate all the mechanisms, allowing us to optimise the
principal dimensions, before then transferring the data into a CAD program to produce the required detailed drawings. All the
required materials were sourced new from our suppliers with the exception that most of the hydraulic components were obtained
second hand in an attempt to limit costs.
We then set about fabricating all the components, making use of our in house machining, profile cutting and welding capabilities.
The control electrics were made up and housed in robust steel boxes and the hydraulics were piped in.
We had sufficient confidence from the design process that it was decided (rightly or not!) to paint everything at this stage, rather
than having to dismantle the machine later.
Main Features
•  Hydraulic supply - taken from the tractors flow and return
lines it passes through a priority flow divider before returning
to the tractor.

•   Bale Clamp- this clamps the bale on both sides of the cut,
allowing clearance for the blade to pass through the bale
whilst preventing the crop from being dragged out. The bale
clamp is actuated by a pair of hydraulic rams via an electro-
hydraulic spool valve supplied from the priority flow.

•   Reciprocating Blade - this is a cantilever arrangement, with
the reciprocating motion provided by a bell crank actuated by
a regenerative ram, which gives equal speed/force in both
directions. An electro-hydraulic spool valve coupled to the
priority supply, and controlled by 2 limit switches and a latching
relay circuit to give reciprocating motion feeds the ram itself.
End-of-stroke shock loading is controlled by a combination of
hydraulic accumulator, rubber bump stops, and fine adjustment of
the limit switches: a combination that gives a degree of energy
recovery in the system.

•   Blade downward motion-this is governed by the application of
pressure to the actuating ram. This pressure can be set by an
adjustable relief valve, which in turn is fed from the surplus flow
side of the flow divider.

•   Pivot Points - it was decided that all main pivot points should
have Teflon coated plain bearings and have a facility to be
lubricated with grease. All other pivots would be pins running
directly on steel housings .
Automated Operating Cycle
Due to the use of electro-hydraulic valves, the system lends
itself to a degree of automation. In this case we decided to use 12v
relay logic mounted on the machine with a cab mounted switch box
to control the cycle. The complete cycle is as follows.
•  The machine is reversed against the bale and clamps around it.
•  The bale is then lifted clear of the ground, ready for the
operator to start the bale cutting process by flicking a switch
on the control box.
•  From now the sequence is automatic, the reciprocating blade
working its way on down through to the base of the bale
where it sets a limit switch. This in turn, shuts off the blade’s
cutting action and returns it to its start position.
Commissioning
For in-house testing, we made use of our own 3 phase hydraulic power pack to
test the machine in the workshop and almost immediately encountered our first
problem: the machine would stop mid-cycle for no apparent reason. After some
investigation, this was traced to a pair of relays unlatching themselves due to
machine vibrations, easily cured by the
installation of capacitors across the coil terminals.

Having passed in-house testing, the machine was then taken on site for testing
on a real bale of hay. The results were mixed...
•  Promisingly, the machine did cut the bale.
•  It did so very slowly.
An attempt to "tune" the machine by altering flow rates and relief pressures
resulted in a small improvement, but it was only when pressure readings were
taken at the hydraulic rams that the real problem became apparent: the rams
were working very close to the maximum system pressure. At this stage, the
decision to produce a flexible design proved well founded. To affect a cure, it
was a simple matter of  increasing the size of both the reciprocating ram and
the downward pressure ram.
Success!! We now had a machine that worked extremely well, and as can be
seen from the pictures, it produces a very clean cut with virtually no effect on the
integrity of the bale. This was an extremely satisfying moment for all concerned,
especially the look of sheer delight on the client's face.  The entire cycle takes
around 40 seconds. We also tested the machine on haylage and straw and
found no degradation in performance.
Follow-Up Service
To date, the machine has proved extremely reliable, and has only required the replacement of 2 faulty relays and a repair to the
saw teeth after trying to cut a tedder tine!!

We have since designed and built one other bale cutter based on the reciprocating principal but is radically different in many
other respects in that it is purely mechanical and is not tractor mounted,
but that’s another story!!
Inspired by this article
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