Offbeat
Engineering
Case Study: Brash Rake
Design Considerations – Governed by availability of stock
components
It was clear that this design would have to be based around the use of readily available
stock components, thus minimising down time in the event of a breakdown. These
components were therefore sourced prior to the main design process commencing.
Strangely, most of them were sourced from the road haulage industry (good tried and
tested stuff).

•   Bonded rubber bushings (metalastic drawbar pivots)
•   Rubber bump stops (suspension limit stops).
•   Brass bushing (spring shackle bushes).
•   Tension springs (rear door counter balance springs).

From here it was a case of designing the rake to incorporate these components. As with
most designs you start off thinking there is plenty of space to accommodate everything but
soon you encounter the problem of interference. The solution here was to write a computer
program to simulate the motion of the moving parts, and therefore optimise the design,
before transferring the principal dimensions into a CAD application to produce the
drawings
Fabrication - From CAD to Metal
We then set about fabricating all the components .The final part of this process was to
modify the skidder unit’s front blade to take the mounting brackets.
What is a Brash Rake
A brash rake is a machine which would normally be mounted on the front
blade of a fairly substantial skidder unit. It's function is to rake up the residue
(brash) left after tree felling operations,.This would then either be burned or
chipped.  

Problem Definition
Scottish Borders timber contractor R.J.Macvicar had for some time been
using a brash rake borrowed from one of his main contractors. In principal
this rake worked well except that it had obviously not been designed for the
very harsh working conditions encountered in the UK. It was an ongoing
process just trying to keep the rake functioning,.Clearly what was needed
was a major rethink about the way that it had been constructed. Having been
involved in much of the repair work we had a very good insight into what was
needed to construct a machine which could stand the pace.

•  It should retain the same basic working principle: 4 spring
loaded retractable tines.
•  The retractable tines should be much stronger.
•  These tines should be mounted on compliant carriers to isolate
the main frame (and skidder) from shock loading.
•  The tine track rollers should have plain bearings which should
be less sensitive to dirt ingress,  unlike ball bearings.  
•  The carrier and spring assemblies should be heavily guarded
against entanglement from branches.
•  The machine should be mounted in such a way as to allow
adjustment of tine approach angle.
Commissioning
The only workshop based testing we could perform was to mount the rake
on the skidder and check the tine movement for interference. Subsequent
testing was carried out in a wood, raking up hardwood brash. Except for
some small modifications to the guards, the rake worked perfectly.

Follow-Up Service
This particular rake has been in service for over 10 years during which
time it has required only the following maintenance:

•   Renew skids on tine bottoms
•   Renew or rebush tine track rollers
•   Renew some worn sections of tine
•   Replace broken/stretched springs

In fact many of these repairs could have been avoided had the operator
kept to the recommended maintenance schedule.
We have since built one other brash rake which is now operated by Frank Ward Timber
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