TW-715 Project

Page 2: Building the Engine and Transmission

   

Time to start the build!  I assumed that all those numbered bags would be used in order, but that turned out not to be true at all.  For example, bags F14 and F21 are needed for the first step as shown.  The instruction videos actually don't even refer to the bag numbers (except for the screws), so you just need to find the right parts based on what they look like.  I suspect the numbers are just there to make it easier to fill out the checklists when packing the boxes.  The very first parts needed are the transmission bell housing and the rear seal, both machined aluminum.  If you zoom in on the photo you can see the cutter marks on the bell housing confirming that it is machined and not cast.  I'm not quite sure why nearly all the engine and transmission parts are clear anodized but the bell housing is black.  Maybe just for some contrast.  7 small cap screws are used to connect them together.


   

The next step is to connect the front housing of the transfer case to the back of the gearbox.  On the left you can see the round indexing feature with one flat side on the transfer case housing which is used to make sure it can only be installed in a single orientation.  There are 4 bearing recesses in the housing as you can see on the right.


   

Here I ran into my first quality problem on the model only three steps in.  The hole for the screw circled in red on the left was not tapped deep enough for the screw to be inserted completely.  With the screw standing proud, the bearing could not be installed flush and therefore the whole transfer case just jammed up once screwed together (which is how I discovered the problem).  I had to manually drill and tap the hole deeper with an M2 tap to solve the problem (corrected on the right).


   

Now we'll build the gearbox output (driven) shaft.  Both of the gears which will eventually be on this shaft are on bearings and therefore can rotate freely and independently of the shaft speed.  Only when the driving ring keyed to the central hex is engaged with one or the other of the gears will the shaft be driven.  The three curved slots seen on the back of the gear on the left will be engaged by dogs on the driving ring.  If the driving ring is centered (not engaged with either gear), then the gearbox is effectively in neutral meaning that the motor can spin freely without any connection to the wheels.  Note that all of the gears within the main gearbox are helical.  Helical gears run smoother than straight cut spur gears but are also noisier.  The whine of this gearbox is very distinctive once you know what to listen for.

This gear is used for high speed operation and appears to have 26 teeth.  I didn't count the teeth before assembling the model so I am relying on pictures (including the instruction video).

I should mention a clear Capo tie-in here.  As far as I can tell, these transmission gears are the identical parts that are used in the Capo 15827 JK Max.  Both gearbox configurations use a layshaft.


   

Now I've installed the output shaft into the gearbox housing.  You can see that it protrudes straight through the transfer case and will be used to directly power the rear drive shaft.  There are four straight cut spur gears inside the transfer case as shown on the right.  The two intermediate idler gears have 14 teeth.  The two larger outer gears have 17 teeth.  None of these tooth counts really matter since everything cancels out resulting in a 1:1 ratio between the front and rear axles.  The gear to the far left of the image is used to power the front drive shaft which, because there an even number of gears, rotates the opposite direction as the output shaft.  A close look at the gear on the far right (on the output shaft) shows that it is not locked to the shaft but has three driving dogs.  This means the transfer case will only power the front wheels when something is locked to this gear.  More on that later.


   

This little shift fork controls the 2WD/4WD selector which is built into the transfer case as described above.  The spring tends to keep the t-case in 2WD unless something is forcing it into 4WD.  Because the driving ring will have to rotate relative to the fork, the fork contains 4 pockets for ball bearings.  I did not install the balls yet at this point due the missing part described below.


   

Here I ran into an extremely frustrating problem.  The driving ring I needed for the t-case selector was not in the kit.  A part which looks quite a bit like it was included instead (packing error).  The error is detailed in the photo I made on the right.  The circled part is what I needed, the boxed part (which is actually for the locking differential) is what I got.  Luckily Sumi and Kai-Oz.net sent the correct part out to me right away, but it meant I had to either stop building or find a way to continue without it.  As shown on the left, I was able to just leave the part out for now which resulted in RWD only.  Because I knew I was going to have to tear down the transmission later to install this part, I didn't grease any of the gears yet (which also makes for better photos).


   

Here I'm installing the t-case rear housing without the driving ring installed.  Once it is buttoned up with the six cap screws shown, you can't even tell anything is missing.  The little hole in the shaft on the shift fork will be used to connect to a mini-servo linkage for controlling the t-case.


   

Now we can start building the primary shift fork.  This works very much like the smaller one with 4 ball bearings in the fork.  This fork is spring loaded to center which means the transmission will be in neutral unless forced into either high or low gear.  Note the six driving dogs on either side of the drive ring which will engage with the three slots in the mating gears.


   

Now I can slide the low speed gear onto the out shaft.  This heavy steel gear has 38 helical teeth and rides on two ball bearings as shown.  Note that it has 3 slots facing the driving ring just like the high speed gear.


   

Now we'll build the layshaft.  The presence of two cross pins tells us that both gears are going to be locked to the shaft.  The gear on the left is the high speed driver and has 26 teeth which means high gear is 26:26 = 1:1.  The smaller gear on the right has 14 teeth which results in a ratio of 38:14 = 2.71:1.  That's a big difference between high and low gear (as it should be for something used on the trails).  The larger gear on the right drives the layshaft and has 31 teeth.


   

Now the layshaft has been installed into the gearbox housing, but we're not done gearing down yet.  Let's another stage of reduction.  The gear shown on the lower left will ride on the output shaft, but not be keyed to it.  The larger diameter 44 tooth gear acts as the motor spur and the smaller diameter 20 tooth drives the layshaft.  The layshaft ratio is 31:20 = 1.55:1.


   
 
The cheap Axial 55T motor shown is just a placeholder until my Holmes Hobbies motor shows up.  Remember, I have to tear down this gearbox to add that missing driving ring anyway.  The helical cut motor pinion has 12 teeth, so the motor drive ratio is 44:12 = 3.67:1.  Let's install the oil pan with four cap screws and then calculate the overall transmission ratios.
 
   

Time to connect a battery and see how this thing works!  It doesn't.  The motor just stalls.  It took me quite a while to figure out the problem and it is shown circled on the right.  The screws which hold the front bearing support to the oil pan are too long and protrude into the cavity.  They then interfere with the adjacent gear and prevent anything from moving.  I'm amazed that this design made it into production because it is impossible to operate the model using these screws.  There were no appropriately shorter screws among the hardware provided so I just cut off about 1.5mm from each screw with my Dremel tool and the problem was solved.


   

In keeping with my tradition of building this thing before I had all of the necessary electronics, I'm using an old seized micro servo as a placeholder until the new ones arrive.  This servo controls the high/low gear selector of the transmission and mounts inside the engine block right beneath the motor as shown.


   

With the shift servo installed, I can start installing the two machined housings which make up the engine block.  As you can see, there are openings on the front and the top.  The slot that I'm currently using for the wires is actually not correct, I should be running them through the top opening to be hidden by the intake manifold.  I'll fix that when I install the correct motor later.  At this point it is possible to connect a battery to the motor and try everything out.  It works just fine apart from the front drive shaft output which is waiting on that missing drive ring.


   

Here are some cool detailed engine parts.  These cylinder bores sit up on top where the heads should be.  This is not strictly accurate of course, but I appreciate the attempt at a scale engine.  There are even water jackets machined around the cylinder bores.  Nice touch!


   

The final couple of details for the engine block are the intake manifold and this photo etched part which bolts to the front.  I don't think it represents any particular part of a real engine (maybe a timing cover), but it does allow some air flow into that enclosed interior to help cool the motor.


   

This might not look like part of the engine and transmission assembly, but because of the way it mounts it appears in the same group of steps.  The center console is a single part with a separate opening lid for the compartment.  There are three levers here: a parking brake lever, a shift lever, and a transfer case lever.  The shift lever will actually be linked to the transmission and indicate the gear position.  I'll be painting all of this later.


   

This little plastic lever (left) protrudes into the bell housing and connects to the shift fork causing it to slide back and forth and indicate the selected position.  It connects to the lower shift lever shown on the right.  Unfortunately, the transmission refused to shift once this section was installed.  It turned out that the sliding plastic lever was too thick and got pinched between the bell housing and the center console, effectively locking out the shift function.  I had to sand down the lever to make it thinner and now it works correctly.




This photo was taken considerably later when I came back to do the painting.  The center console has been painted in semi-gloss black and I've highlighted certain areas with a chrome paint pen.  The parking brake lever and gear selector can be moved even though they don't attach to anything.  The high/low selector functions as described previously.


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