I was mesmerized by a particular haunt prop at the Transworld Chicago show in 2003.
The Scarefactory's Floating Reaper prop was incredible. The motion was fast and wild.
The range of motion was impressive as well. The character would lift and fall several feet while swinging side-to-side.
Scarefactory's Crypt Wraith prop mimics these movements, but hinges on a ground level mechanism as opposed to the
Floating Reaper's elevated mechanism. Since my Halloween display is essentially a graveyard scene, I can't incorporate
elevated machines without building something to disguise or completely cover the structure.
This is my attempt to reproduce the movements of one fantastic haunt prop while keeping the machine as low as possible.
The goal is to build a base/frame that will firmly hold a short vertical post or mast. The outer shell of the mast will rotate roughly
sixty degrees, powered by a pneumatic cylinder. The mast will also support a single lifting arm and a second air cylinder.
This machine will move through a fairly large space.
Considering it will be less than twelve inches tall in it's retracted state, it has to be built "tough."
We'll be lifting and swinging a seven-foot-long lever with the added weight of a prop character.
The motion needs some speed as well - not startle speed, but an angry kind of speed.
Moving parts, making it strong and dependable.
The rotating spindle will be the key to this machine's successful operation.
I chose to fabricate the spindle using a couple of specific main components described in more detail below.
This is certainly not the only way to produce a functional joint with these capabilities, but it is my opinion that this is the
most durable and dependable method. Since this joint is critical to the performance of this machine, a high quality fabrication
is a good investment.
Tapered Roller Bearings
Bearings are used to reduce friction in rotating joints. Reducing friction not only allows the joint to move with
less effort, it also reduces wear on the individual parts of the joint.
Incorporating bearings in the Bellicose machine joints will provide two important benefits.
The leverage disadvantage on the air cylinder will be slightly lessened once joint friction is reduced.
Also, the chance of the joint becoming sloppy over time is practically eliminated with basic bearing maintenance.
(cleaning, visual inspection, lubrication, etc.)
Want to learn more about different types of common bearings and how they work? Start here;
Once you know the load characteristics of your project, you can easily determine the proper bearing for the job.
In our case, we will be working with an axial load and a thrust load in the same joint.
Think of a wheel ... The wheel spins on its axis. This is the axial load on the joint ... the vertical "pressure" of the wheel on the axle.
A ball bearing used here would handle the vertical pressure and allow the wheel to rotate freely of the axle.
If you change the direction of the axle while the wheel is rotating you introduce a side-to-side force. This "thrust" load
would push the wheel in a direction across the axle. A thrust bearing used here would allow the wheel to turn on the axle while
providing resistance to the side load.
Ball bearings could handle our axial load easily, but are not designed to handle the considerable thrust load.
Thrust bearings have little axial load handling capabilities, if any.
A tapered roller bearing is designed to handle both loads simultaneously and efficiently.
Finding tapered roller bearings is no major task. Finding a usable size bearing may require a little more effort.
I found a perfect bearing set on Ebay, and later saw the same kit for sale at Northern Tool. Since then, both links have vanished!
What I found was a wheel bearing kit for trailer axles. They should be available at most auto parts stores.
You need to consider three measurements.
The inner diameter , which would represent your axle size.
The outer diameter, which would be the hub's bearing measurement.
And least importantly depth, or total bearing assembly thickness.
If you look for wheel bearing kits, be sure to look for those that work with straight spindle axles.
Some axles are tapered and/or measure differently for each face of the hub/wheel.
I mentioned "perfect" wheel bearing kits ... perfect in that they measure .75" i.d. and 1.75" o.d.
With these common measurements I can use standard pipe size(s) to build my rotating machine joint!
Here's a cutaway, side view drawing of the pivot mechanism I need ...
(A) and (B) are tapered roller bearing sets. The lower bearing (B) is installed so that the angular face of the cone "holds" the vertical (thrust) load.
The cone direction of the upper bearing (A) opposes that of the lower bearing. A washer (G) and nut will be used above the upper bearing.
By adjusting the torque of this nut, we dictate the tension on the bearing cones/outer races. When properly adjusted, the joint will pivot easily and
any side motion (axial "play") will be eliminated.
(D) is the center post. This will be permanently fixed to the base of the machine.
(F) is the lower bearing stop. It is fixed to the center post (D) The outer diameter of the bearing stop is important because it must support
the bearing's inner race, but cannot come in contact with the outer race. The inner race will remain in a fixed position on the center post,
but the outer race will rotate with the case.
The outer race retainers (E) are permanently fixed within the outer case (C).
These keep the bearing's outer race in proper position, and determine the placement of the outer case above the base of the machine.
Since my bearing sets measure .75" i.d., I know I need .75" o.d. pipe for the center post (D).
My bearing's outer diameter is 1.75", so I need 1.75 i.d. pipe for the outer case (C).
The outer case must be 1.75" i.d. in order to snugly hold the bearing's outer race.
The o.d. of this pipe isn't necessarily crucial here either. A usable (weldable) wall thickness is important.
The lower bearing stop (F) has to be .75" i.d., and should be 1" o.d. to avoid contact with the bearing's outer race.
This determines the required wall thickness of the pipe for part (F).
1" minus .75" equals .25"
Divide .25" in half (half of the total gap per each side of the pipe) and we need a .125" wall thickness.
Easy, right? We need pipe that measures 1" outside, and 3/4" inside ... having a wall thickness of 1/8" for part (F).
The outer race retainers (E) must be 1.75" o.d.
The inner diameter in this case isn't overly critical ... as long as we can slide the finished outer case over the finished center post.
This photo shows the rotating portion of this spindle. In the lower left corner you can see the bearing's outer race is installed in the
outer case. You can also see round plug welds roughly one inch from each end of the outer case. Holes were drilled through the case, and when the
outer race retainer stops were in the proper position, they were welded in place through these holes.
DOM Tubing
If you've used galvanized or "black" pipe in your projects, you may have noticed some structural inconsistencies.
There is always a weld bead on the inside of the pipe. This is no big deal when you're plumbing water, but when you're making bushings or
other mechanical parts, the weld bead could pose a problem. Grinding the weld smooth is easy enough at the end of the pipe,
but what if you need a longer length of smooth walled pipe?
Another issue that may be problematic is accuracy in wall thickness, or inner and outer diameter measurements.
Galvanized and black pipe are not always the same measurement everywhere.
If you have access to a metal lathe, making pins, bushings, etc. is an easy process.
If you want stock tubing that can be purchased in various dimensions that are accurate and consistent,
look for DOM (Drawn Over Mandrel) tubing.
This page;
explains the process(es) used to create DOM tubing as well as the benefits of the finished product.
If you're interested in obtaining DOM tubing and your local metal supplier doesn't carry it, look for websites like this;
I found tubing with usable dimensions for the tapered roller bearings sets I bought.
.75" o.d. X .5" i.d. for the center post.
.75" i.d. X 1.00" o.d. for the lower bearing stop.
1.5" i.d. X 1.75" o.d. for the outer race retainers.
1.75" i.d. X 2.00" o.d. for the shell.
Given these dimensions, you'll notice all of the pipe is 1/8" wall thickness ... thick enough for this project.
This material measured well within "tolerance." You may need to sand the outer diameter of one part
or hone the inner diameter of another. If you have access to metal working machines, this tubing is perfect for this task.
Ground level, building the base
The base consists of three main parts. The spindle base, the rear brace, and the forward brace.
It is designed to accommodate future access to the bearings for maintenance purposes. The forward braces are bolt-on pieces
intended to simplify transport and storage issues.
This drawing shows the individual components of the spindle base. The yellow colored square tubing in the center of the crossed legs
is made of a 1-inch length of 1.25" square tubing. The 1 inch square tubing legs (red) are welded to the faces of the yellow tube.
This 1.25 width is needed to accommodate the head of a 1/2" through-bolt for the spindle assembly.
The square plate (blue) is 1/4" plate stock with a 3/4" hole drilled through the center. This will accept the center post (green).
The center post is welded to the plate from underneath. The weld is ground smooth, and the post/plate assembly is tack welded to the legs
making sure the 1/2" through-bolt is aligned properly. After making sure everything fits properly and checks "square",
the new spindle base assembly is welded 100 percent.
Rear Brace Assembly
Rear braces are added to keep the spindle assembly securely in position.
A 2 X 3 inch piece of 1/4" plate is drilled to accept the through-bolt from the spindle assembly.
2 pieces of 1.25" square tubing, 2 inches long, are modified to work as saddles.
2 pieces of 3/4" square tubing are cut 12 inches long, with parallel 45 degree miter cuts on each end.
The sections of 3/4" tubing are welded to the 1.25" saddle pieces.
The 1/4" plate is bolted to the top of the spindle and the welded brace parts are positioned and marked.
Remove the pieces, tack the rear brace parts to the plate and test the fit. The entire rear brace assembly should
slide straight down into position with ease. If the fit is good, weld everything 100 percent.
Install the brace assembly, bolt the top down, and clamp the saddles to the base legs temporarily.
Drill holes and install bolts through the saddles and base legs to finish the rear brace assembly.
Directly opposite the rear brace assembly, the remaining two legs of the spindle base are joined with a cross brace.
Positively linking these two legs is part of the stability plan for the forward braces. The photo above also shows the
addition of two angle iron brackets. These brackets share matching hole patterns with the forward brace segments.
More on that below ...
Forward Braces
It is essential that the forward braces are strong. When finished, they will be almost seven feet in length. They will
attach to the spindle base an additional twelve inches from the machine's center of gravity. This total distance
must be strong enough to support the weight of the prop body in motion. Although not a requirement, I also decided
to apply a low profile theme to this whole machine. Finally, a wider "footprint" will provide better support on less stable
surface(s), such as my lawn ... Using a truss type system, I am able to accomplish all of these tasks
while keeping the segments smaller in size and lighter in weight.
To keep the pieces small for storage reasons, I decided to break the individual braces in half, each section being 38 inches in length.
I started with ten-foot lengths of 3/8" rebar (reinforcing bar for concrete work.) Each bar was cut into three pieces of equal length.
Four sticks of rebar were needed to make twelve pieces ... enough for four leg segments.
2" X 2" angle stock was cut into five-inch lengths. Ten of these pieces are needed. Carefully mark and drill holes in all of these
pieces before any assembly. Slightly oversized holes will be helpful later on as pieces get mixed up and turned around.
With this setup, any segment can be used in any position, so it's important that the holes are all the same on every piece of angle.
The lengths of rebar are tacked in place between two angle pieces. Check for straight and square, and weld them in place.
The real strength of this design is in the diagonal bracing. I used 1/4" cold rolled round bar for this job.
Bend the 1/4" round bar into shape and tack it to one side. Check for straightness and repeat the process on the adjacent side.
Weld all of the joints 100 percent on the top side, flip the segment over and finish the welds underneath.
While upside-down, I added two cross braces between the lengths of rebar, equal distance from each end.
This design was a little more work than using big flat or tube stock, but it was worth it.
This is very strong, and reasonably lightweight compared to large tube stock.
With the basic form and function of the machine complete, it's time to move into more detailed aspects of the movements.
The spindle assembly will allow the prop to swing left and right. Attached to the outer casing of the spindle assembly is a single
pivot joint. This joint will allow the prop to travel up and down. The decision to use bearings here was based on the same principle
as within the spindle. Reduce friction, and ensure longer life to the joint components.
These are sealed ball bearings. They are designed to handle radial loads. These particular bearings have an 1/8" flange
on one face which are to be seated against the receiving end of the assembly. Without this flange, it would be necessary
to add a backing shoulder, collar, sleeve, etc.between the bearings. The inner diameter of the these bearings is 1/2",
and the outer diameter is 1.180".
Considering the desired strength of this joint, the 1/2" i.d. is perfectly suited to a 1/2 pin. Anything smaller might not be strong enough
if the prop weight and leverage become larger than anticipated. The bearing's outer diameter is slightly larger than the inside measurement
of 14 gauge 1 1/4" square tube. This worked out perfectly. The bearings were pressed in and remain snugly in place.
Two short lengths of 1-inch angle material work very well in providing a strong, square point of attachment between the verical, round spindle
and the horizontal, square lift pivot joint. This approach allows for roughly nine linear inches of weld ... far stronger than the individual components.
Lift Arm Base Section
This is the base section of the lift arm. This mounts to the pivot joint attached to the spindle.
An air cylinder mounted between the spindle and this lift arm will cause the arm to move up and down.
The prop body will be mounted to the opposite end of the lift arm when completed.
1 inch square tube spacers are cut to length. This measurement should match the total width of the
pivot joint's bearings, which protrude slightly beyond the housing. These spacers are clamped to a pair of
1" X 1/4" flat bars. These flat bars have 3/8" holes centered at one end. The pivot pin is installed through these
holes and the pivot joint bearings. When the pin is square (90 degrees) to the flat bars and the whole assembly is flat, tack the
square tube spacers to the 1/4" flat bars. Additional 1 inch square tube bars, roughly 14 inches long, are stitched
to the outside faces of the flat bars. (In the next phase of this lift arm construction, these 1 inch tubes will accept
the rest of the lift arm.)
Check for straight and square, cover the bearings, and finish welding the joints ... top, bottom, and inside the spacers.
This is now a heavy duty component. Depending on the speed used to move the finished prop, this extra level of durability
may come into play. If this is over-built, it won't be a problem. The additional weight here is directly above the lift cylinder and
therefore won't introduce any unnecessary leverage woes with the lift arm assembly.