3D printed engine BOM and assembly instructions

3D printed Stirling engine

This third post on the 3D printed Stirling engine PE2 should cover most of the loose ends for those interested in building one. First I’ll provide the complete BOM (bill of materials) needed to build the engine. The reduced copies I’m showing here are barely readable, but you can download the ful-size PDFs at Thingiverse. After the BOM I’ll discuss finishing the 3D printed parts and assembly.

BOM – Parts

BOM for the PE2 Stirling Engine, page 1 parts

BOM for the 3D printed Stirling Engine PE2, page 2 parts.

BOM – Fasteners

BOM for the 3D printed Stirling Engine PE2. Page 3 fasteners.

Assembly – finishing the 3D printed parts

Drilling out the holes

First a word about safety. Just because you have access to a drill press (or any tool) doesn’t mean you are safe to use it. Read and follow the safety documentation that comes with the tool or get someone competent to instruct you in safe operation of the tool. I’m not able to give safety instruction.

See the PE2 BOM for recommended drill sizes for drilling out the necessary holes for shafts and pins. The printed parts have the holes formed slightly undersized. I recommend using a drill press with the parts held accurately and securely in a drill press vise to make sure the holes are in correct alignment.

The crank pin holes need to be perpendicular to the crank disk faces to avoid binding of the connecting rod as the crankshaft spins around.

Besides fractional and number drills, I recommend the use of two metric drills: 1.65 mm (.065″) and 3.2mm (.126) for proper fits.

My general recommendations for the 1/16 music wire:
1/16″ (.0625) for press fits
#52 (.0635) for light press fits
1.65mm (.065) for slip fits

Cylinder body

Clean out any ABS hairs or other obstructions from the center hole so that the displacer can fit without touching anything. The 12 holes for attaching the hot and cold plates were designed slightly large so they shouldn’t need to be drilled out. The top and bottom surfaces should be flat, but the important surfaces are the inner shelves where the O-rings seal. These need to be flat and smooth to seal properly. If they aren’t you’ll need to sand and fill them until they are or otherwise find a way to seal the hot and cold plates to the cylinder body.

Light clamping force when tightening the machine screws should be used to avoid possibly over-stressing the part. It is only necessary to get a good seal on the O-ring. If you over-tighten the machine screws you could break the cylinder body.

Displacer crank disk

Drill out the holes to the recommended sizes. Note that you’ll want to insert the set screw nut and start the set screw before assembling the displacer crank disk to the flywheel. The crank pin hole on a 0.25″ radius is the recommended hole. The other holes provide slightly longer and shorter strokes for use if the displacer stack is slightly off the basic 1.00″ thickness.

Power piston crank disk

Drill out the holes to the recommended sizes. The crank pin holes provide different stroke lengths for different heat ratios and provide for some experimentation. The shorter stroke lengths are best for lower temperature ratios. I would recommend starting with the 1/2″ stroke ( 1/4″ radius).

Always remove the crank disk to move the crank pin because it requires quite a bit of force. This applies to the displacer crank disk too, although that should not need changing after the initial fitting.

The engine requires a 90 degree phasing between the displacer crank pin and the power piston crank pin. An eyeball 90 degrees should be adequate– just line up one crank pin vertically with the crankshaft and the other one horizontally. The direction of rotation is such that the displacer crank pin leads the power piston crank pin by 90 degrees.

Connecting rods (both displacer and power piston)

Drill out the holes to the recommended sizes. You want easy frictionless rotation, especially on the crank pin end (the end made oversize). If you put the music wire shaft through the hole and turn it, the connecting rod should hang vertical. The other ends of the connecting rods aren’t quite as critical because they only rotate through a small angle, but they must not have much friction either.

When the engine is running, it is very quiet except for any play in the connecting rods. This is especially true for the power piston con rod. I have found that a little white lithium grease applied to the crank pins will help quiet the noise. I don’t think it reduces the friction but it doesn’t add much either.

I have made the crank pin end of the con rods oversize so that they could have a bushing inserted if they get too much play from wear. At this point I have about 50 hours of run time on the engine and don’t believe wear is going to be an issue.

Power piston insert

Drill the wrist pin hole to the size recommended. The wrist pin is long enough that it remains captive even if it were to slide all the way in either direction when it is inside the power piston. The insert needs to seat flat against the bottom of the piston to have proper alignment (wrist pin parallel to the crank pin). The flat head screw that holds the insert in the piston needs to make a good seal with the piston to avoid leaks. This shouldn’t be a problem if the piston is countersunk properly.

Fork

Drill this part out as specified. This part is a little difficult to work with because it is small. When drilling out the displacer shaft hole, be certain to clamp the piece only on the end which does not flex to clamp on the displacer shaft. Then drill it all the way through. When drilling the hole for the pin, I would recommend a slip fit on the side away from the flywheel (same side as the clamping screw). Use a light press fit for the last half of the hole to help hold the pin in position. The goal is to be able to put the pin in easily but have a light press fit the last bit to hold the pin in position.

You won’t normally need to take the pin out. To remove the fork and con rod you can just loosen the displacer shaft clamp, turn the flywheel until the fork is free of the displacer shaft and then the fork and connecting rod slide off the crank pin together.

Flywheel support

It’s a good idea to save this part until you’ve drilled out some of the easier parts because this one is critical. The most critical hole is the displacer shaft hole in the flywheel support base. If this hole is not aligned accurately, the displacer could rub on the cylinder base, causing too much friction for the engine to run. You need a well-aligned slip fit on the displacer shaft without a lot of play. To drill this I clamped it upside down in a drill press vice and carefully aligned it to have the bottom parallel with the drill press table. Start with the #52 (.0635) drill and test the hole with the displacer shaft. This will probably be too tight so you’ll need to go to the 1.65mm drill (.065).

When attaching the flywheel support to the cold plate and cylinder body, you need to insert the two O-rings and then carefully align the flywheel support so that the displacer can slide up and down without touching the cylinder body wall. You should be able to spin the displacer around without touching the walls and run it all the way up and down. Note that the foam displacer pieces can be cut slightly undersize to clear the cylinder body, the ABS displacer part is the one that must not touch.

When tightening the screws that hold down the FW support, you may not be able to fully compress the O-rings so that the base seats against the cold plate. The important thing is to align it for the displacer and get a good seal for the O-rings. Avoid tightening the screws to the point of distorting the base of the flywheel support.

The hole for the bearing tube in the flywheel support is a little undersize. I was reluctant to drill it out because it is a split hole and the powered drill might grab and rip the piece apart. I cleaned it out by holding a 1/2″ drill bit in my hand and twisting it in the hole. You might also try a round file.

The power cylinder clamp was a good fit around the power cylinder. The O-ring grabs the power cylinder adequately so I would only use the power cylinder clamp for extended inverted operation.

Both of the above clamps should be tightened carefully so that the ABS is not over stressed.

Flywheel

You may have to do a little fitting to mount the flywheel to the flywheel crank disk.

Displacer

The printed part of the displacer uses a nut for the set screw that holds the displacer to the shaft. The other six holes are drilled (#43, .089″) and tapped 4-40. Captive nuts were not used for these holes to minimize weight.

I made an aluminum template to accurately lay out the center and 6 holes. This made it easy to form the 4 similar parts.

Power cylinder and power piston

The brass tube 1″ OD x .065 wall is cut to length (or ordered to length for a single part) and the ends are faced square and chamfered.

The power piston can be made from either an aluminum rod by boring out to .75″ diameter or use a tube and plug the end (either press fit or epoxy). I used a .875 dia x .065 wall tube and press fit a plug.

Turn the power piston to have about .001″ clearance in the brass tube. The piston should fall freely in the brass tube when held vertically.

When the hole in the piston is plugged and the brass tube is held vertically against a surface that seals, the piston should fall very slowly under its own weight– around 10 seconds or more to fall one inch. You can also use this method when the engine is assembled without the con rod attached to the crank pin to test for leaks in the engine. There will be more leaks (mostly around the displacer shaft), but the piston should still take about 4 seconds or more to fall one inch.

I have not found any lubricant to be useful between the piston and the cylinder. The friction is almost zero in the vertical orientation which is how the engine should always be operated.

It is very easy to install and remove the piston assembly if you do it correctly. Here’s how:

(1) Assemble the piston with insert, wrist pin and con rod and hold it inside the power cylinder.
(2) Bring the con rod end up to the crank pin and slide it over the crank pin while the cylinder and piston are angled to the side.
(3) Turn the flywheel and crankshaft until the crank pin is at its furthest point from the cold plate. The piston and cylinder will now clear the cylinder clamp and can be rotated and cylinder lowered inside the cylinder clamp.
(4) Apply a downward twisting motion to the power cylinder to push it through the O-ring and seat it against the cold plate.

To remove the piston assembly just reverse the instructions. The only force required should be to push or pull and twist the power cylinder through the O-ring.

For a long time I had problems with water condensation on the power piston and cylinder. When enough accumulated the friction would stop the engine. Removing the piston and cylinder assembly and wiping the water off the piston and cylinder with a soft cloth would restore operation. This problem persisted for quite a while until the ABS was finally dried out. I haven’t had the problem any more although it might return if I don’t run the engine for weeks.

I would suggest trying to dry the cylinder body and displacer at elevated temperature if you have this problem. Of course you need to be careful not to exceed a safe temperature for the plastic which for ABS should be about 170 degF. I would think drying it at around 130 degF should be adequate.

Music wire shafts and pins

I polish the music wire before I cut it to length. This is particularly useful for the displacer shaft and the crank pins to reduce friction and slow the wear of the ABS. This step is certainly optional. I do recommend the use of white lithium grease or another light grease suitable for steel and ABS on the crank pins where the connecting rods turn and also on the displacer shaft where it slides through the guide in the flywheel support.

Bearing tube

The plastic bearing tube as printed is slightly oversize on the OD and undersize on the ID. This makes it tight when the R2 bearings are inserted and I was concerned about over-stressing the bearing tube clamp on the flywheel support by trying to open it too wide. I hand turned a 3/8″ drill inside the tube until the bearings would slide in easily and then worked the outside of the tube with a file until it would fit in the bearing tube clamp without stretching it open much. If any filing is needed it should probably happen only to the outside of the bearing tube to maintain the best alignment of the bearings. Although I prepared to use a split aluminum tube as a backup to the plastic, I think that the plastic is working satisfactorily.

Bearings and crankshaft

The R2 ball bearings (1/8 bore, 3/8″OD) provide low friction regardless of the flywheel weight. I used ground and polished stainless steel shafting for ease in sliding on the bearings, but any straight steel shaft will work as long as the bearings will fit.

Only a light force is needed on the bearing clamp of the flywheel support to lock the bearings in place.

Fasteners

The BOM lists the fasteners I used. All flat head screws are for 82 degree countersink. Although I used socket head fasteners, other heads may be used.

Regenerator

This engine will operate without any regenerator, but it will run much better with a regenerator. Learn how to add the regenerator here.

Running the engine

If you build this engine according to the plans and instructions, it will run. There are only three things that will prevent it from running:

1. Not a high enough temperature ratio. If the cold plate is around a normal room temperature the hot plate needs to be about 50 degF hotter. You can always use more as long as you don’t go above 170 degF and deform the plastic. When initially trying to get the engine running, you can also place an ice cube on the cold plate to create more of a temperature differential.

2. The engine leaks too much. Test for leaks by placing the power piston (con rod not connected to the crank pin) in the power cylinder. It should take at least 4 seconds to fall one inch. If it falls faster then you need to find out where the leaks are and fix them.

3. Too much friction. This engine does not produce much power so it can’t turn with much friction. If you spin the flywheel without compression (remove the power piston or take the hot plate off so that there is no compression) then it should slow down very gradually before it stops. There are lots of possible sources of friction. The most obvious is the displacer rubbing on the side. Align it so that it doesn’t rub. Less obvious sources of friction are binding caused by poor alignment of the crank pins or power piston insert pin. You can relieve binding by drilling the con rod holes slightly larger, but expect a little noise. The displacer shaft could also be too tight in the shaft guide. That would require a little drilling out too.