Stirling Engine Bengs Kit "Laura"

This page last updated: 14 December 2016.


Introduction

Disclaimer: I executed this project in order to make use of equipment that is new to me, build up my workshop and re-learn machining.  The practices I adopted aren't perfect, may not even be good, they just worked for me. Now that's said...

Having bought an Emco Unimat SL lathe a few years ago, I'd been looking for a starter project for it.  At lunch John had already suggested making a Stirling Engine and, while bored waiting for someone in China to fix the [non source-code part of the] driver for the QHY guide camera used in my astrophotography project, I found the perfect thing: a gamma configuration Stirling Engine kit made by Patrick Bengs in Germany, called Laura, in which no milling is required but some machining is required.  In retrospect I could have picked an easier starter project: Stirling Engines require exacting tolerances (0.02 mm on the working piston), no friction and utterly perpendicular joints.  Construction took 4 months of "evenings when I found the time".

Note: the flywheel in this project doesn't fit in the Emco Unimat SL lathe as an 82 mm turning diameter is required and the SL only has 70 mm; the MK3 has 92 mm so would be OK I think. I was originally going to get a friend to turn this part for me but instead decided to purchase a Warco Super Mini lathe as it has a maximum turning diameter of 180 mm (100 mm over the cross slide) in the smallest possible package and will be useful for future projects.

Here are all the parts, as they arrived:

Bengs Laura kit of
          parts

The first thing worth mentioning is that the kit requires some very specific drill sizes, in 0.1 mm steps.  Since I also lacked the necessary metric reamers and metric taps/dies, I ended up spending around three times the price of the kit on tools (also bought from Bengs for convenience).  Then I went way over the top and bought a cheap-yet-huge band saw capable of cutting metal (definitely not required for this project), so that I can get a nice clean cut. Expensive but my workshop is now well kitted-out.  A complete list of the tools I actually used can be found at the bottom of this page.

Making The Parts

I made a list of all the interesting parts from the Bengs drawings and what had to be done to make each before assembling the whole thing.  The list is below, with the result after the doings shown on the right.  The parts were addressed in the following groups/order:
Item
Description
Supplied Material
To Do
Result
1
Base plate: 95.5 mm x 42 mm pre-milled brass plate.
Bass plate, both sides
(both sides shown)
Deburr, drill 2.2 mm holes, drill & tap M2 holes (1.6 mm drill).
Base plate completed
2
(2 off)
Junction plates: 14 mm x 7 mm pre-milled brass plate.
Junction
                  plates
Deburr.
Junction plates completed
3
Cylinder holder, front: 73 mm x 41 mm pre-milled brass plate.
Cylinder
                  holder, front, both sides
(both sides shown)
Deburr.
Cylinder holder, front, completed
4
Cylinder holder, back: 73 mm x 42 mm pre-milled brass plate. Cylinder
                  holder, back, both sides
(both sides shown)
Deburr, drill & tap M2 hole (1.6 mm drill).
Cylinder holder, back, completed
5 & 6
Main bearing support & cover: 24 mm x 26 mm pre-milled brass plate.
Main
                  bearing supports
Deburr, drill 2.2 mm holes, drill & tap M2 holes (1.6 mm drill). Note: drill 2.2 mm holes then clamp top and bottom halves and drill 1.6 mm holes through 2.2 mm holes to ensure alignment.
Main bearing supports completed
7
Crank shaft: 6 mm diameter, 38 mm long, silver steel rod.
Crank shaft
6 mm diameter, 40 mm long, silver steel rod.
Turn down to required length.
Crank shaft completed
8
Crank arm, work: 19 mm diameter, 4 mm thick, brass rod.
Brass
                  rod for parts 8, 23 and 26
22 mm diameter, 46 mm long, brass rod (also used to make parts 23 and 26).
Turn down original material to 19 mm diameter then cut disk free. In the lathe, drill (5.7 mm drill) & ream 6 mm hole. Very carefully drill & tap M3 hole (2.5 mm drill), using a center drill to start it off so that the drill doesn't drift.  Drill & tap M2 hole (1.6 mm drill), cut and file the piece to shape.
Crank arm, work, completed
9
Crank arm, displacement: 25 mm diameter, 4 mm thick, brass rod. Brass rod for parts 8, 24, 25, 27, 31 and 32
30 mm diameter, 101 mm long, brass rod (also used to make parts 24, 25, 27, 31 and 32).
Turn down original material to 25 mm diameter then cut disk free.  In the lathe, drill (5.7 mm drill) & ream 6 mm hole. Very carefully drill & tap M3 hole (2.5 mm drill), using a centre drill to start it off so that the drill doesn't drift. Drill & tap M2 hole (1.6 mm drill), cut and file the piece to shape.
Crank arm, displacement, completed
10
Crank bolt, work: 3 mm diameter, 15 mm long, silver steel rod.
Silver steel rod for parts 10, 11, 13 and 21
3 mm diameter, 200 mm long, silver steel rod (also used to make parts 11, 13 and 21).
Turn down to 2 mm diameter on one end to cut M2 thread, cut to length required, cut M3 thread on the other end.
Crank bolt, work, completed
11
Crank bolt, displacement: 3 mm diameter, 20 mm long, silver steel rod.
Silver steel rod for parts 10, 11, 13 and 21
3 mm diameter, 200 mm long, silver steel rod (also used to make parts 10, 13 and 21).
Turn down to 2 mm diameter on one end to cut M2 thread, cut to length required, cut M3 thread on the other end.
Crank bolt, displacement, completed
12
Connecting rod joint: 6 mm square section, 12 mm long, brass rod.
Brass
                  square section for parts 12 and 18
6 mm square section, 42 mm long, brass rod (also used to make part 18).
Drill (2.8 mm drill) & ream 3 mm hole (part 10 should turn smoothly inside it), cut to size, drill & tap M3 hole (2.5 mm drill), file to shape.  Note: use a centre drill to start each hole so that the drill bit doesn't drift. Connecting rod joint completedConnecting rod joint completed
13
Connecting rod:  3 mm diameter, 58 mm long, silver steel rod.
Silver steel rod for parts 10, 11, 13 and 21
3 mm diameter, 200 mm long, silver steel rod (also used to make parts 10, 11 and 21).
Grind end flat and VERY CAREFULLY drill 2 mm hole as centrally as you can, using a centre drill to start the hole.  If it goes wrong you've got enough material to cut off your mistake and try again three times. Cut to length required and cut M3 thread on the other end.
Connecting rod completed
14
Working piston joint: 5 mm square section, 8 mm long, brass rod.
Working
                  piston joint
5 mm square section, 22 mm long brass rod.
Drill & tap M2 hole (1.6 mm drill), drill (1.8 mm drill) & ream 2 mm hole, cut from original material, cut either side of slot with hacksaw then file down the remainder with a 1 mm needle file.  Don't try drilling it out, such tiny drills wander too much.
Working piston joint completedWorking piston joint completed
15
Working piston: 10 mm diameter, 10 mm long, gun metal rod.
Gun metal
                  rod
13 mm diameter, 42 mm long, gun metal rod.
Turn 10 mm outer diameter, taking great care to ensure a perfect fit in part 26 (get it to within 0.5 mm, then take off the remainder with abrasive paper, test fitting into part 26 frequently, then later put metal polish on the piston and run it through to achieve a final finish) turn 8 mm inner diameter, 8 mm deep, drill 2.2 mm hole, cut from original material.  You have enough material for three goes if the fit turns out wrong; see running.
Working piston completed
16
(2 off)
Driving rods:  6 mm x 35 mm pre-milled, brass plate. Driving
                  rods Deburr.
Driving rods completed
17
Driving rod bush: 6 mm diameter, 11 mm long, brass rod.
Driving rod
                  bush
6 mm diameter, 27 mm long brass rod.
In the lathe drill a 2.8 mm hole (using a centre drill to start the hole off) then ream out to 3 mm, very carefully turn down one end, cut from original material, very carefully turn down the other end. The ends should fit tightly inside the larger holes in parts 16.  Part 11 should turn smoothly inside this part.
Driving rod bush completedDriving rod bush completed
18
Cross piece: 6 mm square section, 6 mm long, brass rod. Brass
                  square section for parts 12 and 18
6 mm square section, 42 mm long, brass rod (also used to make part 12).
Drill (2.8 mm drill) & ream 3 mm hole (part 21 should fit inside it), drill & tap M2 hole (1.6 mm drill), cut from original material.  Note: be generous with the length along the axis of the M2 hole as a bolt will need to bite on that thread without stripping it; it can be filed down later. I used a 4 jaw chuck and did all this in the lathe. If you use a drill press, use a centre drill to start each hole so that the drill bit doesn't drift. If it goes wrong you have enough material for a few attempts
Cross piece completedCross piece completed
19
Piston rod guidance: 21 mm x 5 mm, pre-milled, brass plate.
Piston rod
                  guidance
Deburr. Consider drilling the centre hole out to 3.3 mm (see assembly).
Piston rod guidance completed
20
(2 off)
Guidance bolts: 5 mm diameter, 37 mm long, brass rod.
Guidance
                  bolts
5 mm diameter, 100 mm long brass rod.
Cut two 40 mm lengths of rod, turn down to 37 mm long, turn down ends to 2 mm diameter and then cut M2 threads.  Make sure that the shorter threaded ends are quite square, introduce no distortion while cutting the thread.
Guidance bolts completed
21
Piston rod: 3 mm diameter, 70 mm long, silver steel rod.
Silver steel rod for parts 10, 11, 13 and 21
3 mm diameter, 200 mm long, silver steel rod (also used to make parts 10, 11 and 13).
Cut to length and deburr.
Piston rod completed
22
Displacement bolt: 12.5 mm diameter, 45 mm long, aluminium rod.
Displacement bolt
14 mm diameter, 52 mm long aluminium rod.
Turn down to intended size, drill (2.8 mm drill) & ream 3 mm hole, cut from original material, shape the end with an EMCO Unimat SL1000 radius turning attachment, drill (2.5 mm drill) & tap M3 hole.
Cutting the hemisphere on the displacement bolt Displacement bolt completed
23
Cylinder cover, work: 22 mm diameter, 4 mm thick, brass rod.
Brass
                  rod for parts 8, 23 and 26
22 mm diameter, 46 mm long, brass rod (also used to make parts 8 and 26).
Turn ends to size (I used a parting tool to cut the inner edge), cut from original material, drill 2.2 mm holes.
Cylinder cover completed
24
Cylinder cover, displacement: 30 mm diameter, 10 mm long, brass rod. Brass rod for parts 8, 24, 25, 27, 31 and 32
30 mm diameter, 100 mm long, brass rod (also used to make parts 9, 25, 27, 31 and 32).
Cut from original material, turn the end with the larger diameter sticky-out bit and then, without unclamping, drill (2.8 mm drill) & ream 3 mm hole so as to ensure the hole is exactly perpendicular to this side, then turn the other end to size and make sure that the main disk is no more than 2 mm thick, mark hole positions using parts 3 and 4 as a template, drill 2.2 mm holes, drill & tap M2 holes (1.6 mm drill).  It's really critical to get the M2 holes in exactly the correct position and the central hole exactly perpendicular if all is to be aligned and the displacement piston held centrally when assembled (see running).
Cylinder cover, displacement, completed
25
Displacement cylinder: 30 mm diameter, 27 mm long, brass rod. Brass rod for parts 8, 24, 25, 27, 31 and 32
30 mm diameter, 100 mm long, brass rod (also used to make parts 9, 24, 27, 31 and 32).
Turn outer pattern, separate from original material, drill inner with a 6.5 mm drill and then bore out to correct diameter, drill & tap M2 holes (1.6 mm drill), using part 4 as a template for the thicker end and part 27 as a template for the other end. Use a bottom-tap and be careful not to strip the threads when the tap hits the rest of the material.
Displacement cylinder completed
Displacement cylinder completedDisplacement cylinder completed
26
Working cylinder: 22 mm diameter, 27 mm long, brass rod. Brass
                  rod for parts 8, 23 and 26
22 mm diameter, 46 mm long, brass rod (also used to make parts 23 and 26).
Turn outer pattern (don't go deeper than specified as you will foul the M2 holes), turn down for the front (this should fit into the smaller of the two holes in the cylinder holder plates), drill (a 6.5 mm drill and then a 9.5 mm drill) & very carefully ream (by hand with the reamer fixed in the lathe) 10 mm central hole (ensuring the smoothest possible surface), separate from original material, drill 3 mm hole, make 2 mm bite out of back using grinder and then a round needle file, drill & tap M2 holes (1.6 mm drill) using a bottom-tap and being careful not to strip the thread in the shallow holes (they need to be 3 mm deep on the back and 6.5 mm deep on the front).  The holes in the front can be marked by pushing the part into the cylinder front plate, aligning the 3 mm hole and scribing through the other holes.
Working cylinder completed
Working cylinder completedWorking cylinder completed
27
Glass tube muff: 30 mm diameter, 3 mm long, brass rod. Brass rod for parts 8, 24, 25, 27, 31 and 32
30 mm diameter, 100 mm long, brass rod (also used to make parts 9, 24, 25, 31 and 32).
Turn inner (the hole sized to be 0.2 mm wider in diameter than the measured glass tube and the diameter of the step so that one of the thick seals provided just fits inside it), separate from original material, drill 2.2 mm holes. I used the left-over piece of glass tube to check the size of the hole.
Glass tube muff completed
28
Glass tube: 15.5 mm diameter, 41 mm long, glass tube.
Test tube
(2 off supplied)
Cut to length.  I marked the desired length, leaving ~3 mm extra to allow for finishing, with an indelible pen and then cut the glass with a tiny tile cutting diamond disk intended for use in a Dremmel, held in the lathe grinder but in an extension piece so that I could accommodate the required length of glass tube.  Once cut, finish the end of the part on the same Dremmel diamond disk.
Cut edge before finishing Glass cut edge after finishing Glass tube completed
29
Flywheel: cast/milled steel.
Flywheel
Clamp into lathe at the axle, turn the outer edge and the visible rim/axle to nice shiny steel, drill & ream 6 mm hole (5.8 mm drill), turn the piece around and turn the other rim/axle to nice shiny steel, in a drill press start the angled M3 hole with a chunky centre drill then drill & M3 tap it (2.5 mm drill).
Flywheel completedFlywheel completed
30
(6 off)
Columns: 8 mm diameter, 14 mm long, aluminum rod.
Aluminimum
                  rod for columns
8 mm diameter, 151 mm long aluminium rod.
I worked in batches of two: cut a 30 mm length, turn outer of two parts, cut into two, drill (1.6 mm drill) centre hole, trim ends to length, M2 tap the centre hole half-way from one end and then half-way from the other end.
Columns completed
31
Burner bottom: 30 mm diameter, 20 mm long, brass rod.
Brass rod for parts 8, 24, 25, 27, 31 and 32
30 mm diameter, 100 mm long, brass rod (also used to make parts 9, 24, 25, 27 and 32).
Turn outer, use the parting tool you used to cut the slots in part 25, but mounted at a 45 degree angle, to cut the slot for the rubber seal, separate from original material, drill 6.5 mm hole to 18 mm depth (being careful not to breach the base by accident) and then use a boring bar to complete turning of inner.
Burner bottom completedBurner bottom with rubber seal fitted
32
Burner cover: 30 mm diameter, 16 mm long, brass rod.

Brass rod for parts 8, 24, 25, 27, 31 and 32
30 mm diameter, 100 mm long, brass rod (also used to make parts 9, 24, 25, 27 and 31).
Turn outer (the outer portion is at an angle of 13.25 degrees from the horizontal and the inner portion at an angle of 16 degrees from the vertical), drill centre hole, separate from original material, turn inner, drill 1.5 mm hole. Burner cover completedBurner cover completed, underside

With the parts all roughly made, and after a test fitting to make sure the basics are good, I took some time to finish off the surfaces on all of them:

The steps are shown below. You could, of course, insert more steps of finer abrasive paper before the last step to get a true mirror finish, if you have the patience.  Round parts can be finished, carefully, in the lathe.

Original rough cut
Original cut sides

After diamond filing and initial sanding
After diamond needle filing and initial abrasive
After finer abrasive paper and wire wool
After finer abrasive paper and wire wool

After using a leiderfeile with buffing compound
After polishing with a leiderfeile and compound

Once assembled (you could do this earlier) I gave everything a rub over with metal polish (Autosol metal polish, available from Bengs, is good).

Note: I put off doing more than the first stage for parts 1, 2 and 4 as they will get messy during the soldering process.

Assembly

The Bengs instructions are relatively light on how to go about this, it should largely be obvious.  For the soldering of the base I used a paste that contains both solder and flux, then simply applied a blow torch; definitely the best way to go about it.  Make sure all the parts fit correctly, apply the paste inside the joints, put the parts in position and then heat with the blow torch. I didn't clamp the pieces, relying on the paste to hold things together.  I suggest keeping a couple of sticks of wood around to gently apply pressure if anything comes adrift while soldering.  In any case, the base itself warped in the heat and so straightening was necessary anyway (gently, using a vice to hold the assembly).  The joints are pretty good and it certainly isn't going to come apart [edit: famous last words, see below].

Base assembled
The joint
The joint
The joint

The distortion
After some bending
After buffing After buffing

Other assembly tips, based on my experience:

Here are a few more views of the machine coming together:

Cylinders
                attached
Cylinders
                attached
Cylinders
                attached
Assembling the
                working crank and rod
Assembling the
                displacement crank and rods
Cylinders
                attached
Axle
                assembly (without flywheel)

Glass tube fixed in place
Burner assembled Wooden base added
Assembled

This is what the whole thing looks like:

As you can see, there are plenty of improvements that could be made to the finish but this is my first attempt to machine something since I left school.  This is how freely it runs (the Bengs instructions suggest it should run for five revolutions when "activated"):

Running (1)

In my first attempt to run the engine I used lamp oil, since that is what I had to hand.  It was rather smoky.

The engine did not run so I rubbed off the soot and tried once more but this time using methylated spirit, just in case the flame temperature has an influence; no soot but still no running.

Next thing to try was making another piston as my first attempt was loose, as you might be able to see below.  Bengs says the clearance needs to be 0.02 mm.

That helped but it didn't help enough and, on further investigation, I found that the aluminium displacement bolt was rubbing against the edge of the glass tube.  I polished the surface of the bolt and filed down the edge of the glass tube to ensure it didn't catch but still no dice. I started to wonder if the central hole in the working cylinder was not as properly finished as it should be, since the motion of the piston was not smooth, and then, in attempting to remove part 23 (the brass cover of the working cylinder), I managed to break the solder joint that is holding the base together; Loctite 574 is strong stuff.  Since I needed to take the whole machine apart to re-solder, I took a closer look at all the potentially binding parts:

Working cylinder detail
Working cylinder
Working piston detail
Working piston
Displacement cylinder cover detail
Displacement cylinder cover bore hole
Displacement bolt detail
Displacement bolt
Glass tube detail
Glass tube

The working cylinder and piston should be as smooth as the proverbial, as should the hole in the displacement cylinder cover and, of course, the hole in the displacement cylinder cover should be absolutely perpendicular to the face shown if all the parts are to line up.  So I had another go at making the displacement cylinder cover (part 24, buying some more brass rod to do so), revisited the bore in the working cylinder (part 26) and made another working piston (part 15, using up my last chance with the gun metal) to fit the revisited bore.  The aluminium displacement bolt can rub against the edge of the glass tube so I put some further effort into polishing it, and the displacement rod, to reduce friction.

Working cylinder after some spit and polish
Working cylinder,
improved bore finish

Working piston, final attempt
Working piston,
final attempt

Displacement cylinder cover MK II
Displacement cylinder cover MKII,
bore hole

Displacement bolt polished
Displacement bolt,
more polished

Glass tube polished edge
Glass tube,
polished edge


When reassembling the machine I had immense trouble getting the solder joint of the base to hold.  I ended up switching to another brand of solder paste, leaded and hence with a lower working temperature (180 C as opposed to 650 C), delivered through a syringe, which worked really well: clean the joint, flux inside the joint, assemble the parts, apply solder paste along the outside of the joint, heat the surrounding metal and then, when the flux is sizzling, heat the solder paste and it will be sucked into the joint.

Running (2)

During reassembly I made the effort once again to line up, parts 19, 20, 21 and 22 inside parts 25 and 27, i.e. all the displacement cylinder bits, having taken the time to re-make part 24 with a perpendicular hole.  I looked at parts 21 and 22 inside part 25 and rotated parts 21 and 22 until they were central and so not in contact with the glass tube while running.  The video on the left below shows how freely it runs when activated, which I think is slightly better than before, and the video on the right shows what happened when I tried to run it with a flame; definitely better but still no banana.




One thing I noticed: if you turn the sound up and play the video below, you can hear that there is a hiss of air at some point in the stroke, I think when the displacement cylinder fills the glass tube:



Now of course there should be no air escaping from the system. I tried taking off the glass tube, wrapping my lips tightly around the displacement cylinder and blowing into it: there was no sign of escaping air and the working cylinder moved in response [edit: but of course, I was holding-on to part of the displacement cylinder while doing this, see below]. So I guessed the escape must be from the seal around the glass tube and added some Loctite 574 to the end of the tube itself.  The hiss was still there but you can see if you play the video below that I'm getting really close now:



I separated the working piston arm and checked the action; if you play the video below you can hear that the piston causes a hiss (which is elsewhere on the machine), so the piston is sealing well.



By pushing the piston in and out and using my fingers to detect a draft I determined that the displacement cylinder (part 25) was not sealed properly to the brass plate (part 4), so I took it apart and resealed it.  Now I could feel proper resistance as I moved the piston.  And, finally, hey presto, after exactly 4 months of pottering around, a working Stirling Engine!!!!!



...and in slow motion (x10):


Tools Used

The tools I used in this project were:


Back to Meades Family Homepage