Monday, 17 September 2012

Update and Lathe Works

The postings detailing the experiments with the Shear Coaxial Injector are now ready and will follow in due course. Over the last few months I have been concentrating on setting up the new workshop. This is now 90% complete. Work on the engine will recommence very soon. The new workshop will be a far more comfortable environment to be in as the winter approaches.

Obviously part of setting up the workshop was installing the Harrison M250. During functional tests I found that there was an issue with the torque limiter driving the power shaft. Should the saddle or cross slide be obstructed in any way, causing an over torque condition, this device protects the power shaft and saddle from damage by slipping.

Though not strictly an engine related issue, I have decided to post the details of this problem and the solution here. As a member of the Yahoo Harrison Lathe Group, I hope that others will find the information given useful.

The problem was that stopping the saddle with moderate hand pressure was enough to disengage the torque limiter. The torque limiter works by means of two spring loaded balls engaging with slots on the feed gearbox driving feature. Corresponding holes on the power shaft driven member transmit the drive to the power shaft. An over torque causes the balls to ride around the driven member, coming out of their detent position and thereby disengaging the drive.  On rotating the power shaft with a suitable spanner I could hear and feel the balls disengaging at a very low torque. I surmised the problem was due to insufficient spring loading. This despite the fact that the unit had operated perfectly since I bought the lathe in 2009.

Throughout this exposition I will give Harrison part numbers of the components described in brackets. The spring loading is produced by a pack of 12 Belleville washers in the torque limiter housing (D102H3004). I have often seen Belleville washers fracture, so that was my initial thought as to the cause of the fault.

First of all the torque limiter housing was separated from the feed gearbox driving feature (903013) by removing the two countersunk M4 securing screws. The torque limiter housing was then carefully slid along the power shaft. The two balls were removed from the slots in the driving feature. Below is the driving feature showing the distance piece (D001H2082) and the power shaft partially withdrawn from the latter assembly. The two ball engagement slots can be seen:-



In order to slide the torque limiter housing assembly off the power shaft it is necessary to fully withdraw the power shaft from the feed gear box driving feature. The tailstock end of the power shaft is borne in a steel top hat section bush (906008) which runs in a bore in the power shaft/leadscrew support bracket (906001). This bore is sealed by a plug (906009) that is a transition fit in the support bracket. After slackening the M5 securing grub screw it was possible to withdraw the top hat bush:-  



There was now sufficient latitude to get a punch in to drive out the transition fit plug. A copper drift would have been ideal to prevent damaging the plug. I placed a pad of insulating tape on a standard punch to achieve the same effect:-



Here is the plug partially removed:-




Once the plug is out the power shaft can be withdrawn and the torque limiter housing removed. The housing has an adjusting barrel nut (D021H3001) threaded in the end. This is used to load the Belleville washers and can be removed using a suitable peg spanner or two punches inserted into the driving holes. On dismantling the torque limiter I discovered that the Belleville washers were not fractured. Rather they had been assembled in the incorrect orientation.

 Whilst I appreciate that Belleville washers can be installed in different orientations to affect a coarse adjustment, I do not think this was the case. The washers had been installed in a random fashion. I do not think this would have been done by Harrisons so I can only assume it was the work of the previous owner. Here is the disassembled and cleaned torque limiter assembly:-



The picture shows the housing, driven feature (903028) 6 mm dia. balls x 2, M4 countersunk securing screws x 2, threaded adjuster with locking M4 grub screws x 2, all 12 Belleville washers and the transition fit plug.

The Belleville washers were in good condition. The driven feature abuts the step that can be seen inside the housing. When the washers are slid over the driven feature and the adjuster is threaded in, they effectively spring load the unit.

The washers were next assembled to the driven feature in the correct orientation:-




The complete unit was then reassembled ready to be slipped back on to the power shaft:-



This rebuilt unit was slid back on to the power shaft and reassembled to the feed gear box driving feature. A blob of grease on each ball helped them stay put in the driving features' slots. The tailstock end top hat bush was reinstated and the transition fit plug refitted. 

A full and successful test of the surfacing and facing power feeds ensued. For the life of me, I cannot understand how the unit had functioned for so long, given how it had been assembled.

And here is the complete article:-



The next postings will detail the results of the Shear Coaxial Injection experiments. Stay tuned.

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