Summary: Ego uses a proprietary spline shaft drive system, therefore, non-compatible with existing universal trimmer attachments
Purpose: Illustrate the technical differences between the Ego spline drive and the universal trimmer attachment square drive
Idea: Ego should produce an adapter which allows universal trimmer attachments to be used on the Multi-Tool Power Head (PH1400)
Why: People have garages and tool sheds full of the universal trimmer attachments THEY need for their specific home/yard. An adapter would allow consumers to get into the electric outdoor power equipment (OPE) market with a smaller initial investment. A secondary benefit is that Ego has time to design/build/market "improved" version of the existing universal attachments.
Universal Trimmer Attachments seem to have been around forever, and according to the US Patent Office, an approximate date would be 2004 or 2008 (reference 1). Exact date is not important, the point is that these universal attachments have been around for a long time; and a lot of people have a large useful collection of the tools they need for their home/yard situation.
S1) drive direction: spins CCW (counter-clockwise) when you look from the attachment end, into the power head. Additional details in Reference 2.
S2) quick release (button), secured by a clamp
D1) drive method
Ego: spline drive, 7 flute, 0.270" O.D. (6.85mm ?#!); solid steel shaft 0.280" O.D. (7.11mm)
Univ: square drive, 0.200" (flat side), 0.282" O.D.; flexible twisted cable drive
Note: spline drive could handle more torque, but who's heard of a square drive getting sheared; but the twisted cables have broken.
D2) tube diameter & material
Ego: 0.985" (25.02mm), aluminum tubing
Univ: 1.000" (25.4mm), steel tubing
Note: precludes putting a Univ shaft into a the Ego clamp without modification; weight difference likely negligible
D3) bearing method
Ego: sealed bearing, 6900RS (10mm x 22mm x 6mm)
Univ: flexible shaft, riding in nylon tube, supported by rubber inserts
Note: the sealed bearings are lower maintenance and more efficient, but cost more to produce
D4) clamping details
Ego: 3.125" (~80mm) clamp interface length
Univ: 1.75" (~45mm) clamp interface length
Note: Ego has almost double the clamp length, which will help with rigidity of extended pole saw operations
References and Links:
Phase 2: Figure out how to connect the two different drive systems. My first attempt will be using 3/8-16 Nylon 6/6 threaded fasteners, connected in the middle with a coupler. The Ego side needs to be a male spline drive, and the Universal side needs to be a female square drive. I chose Nylon so that I can drill and get the sizes close, then I'm going to heat it (~500F melting point), and see if I can't get the nylon to mold to the correct spline/square shapes. Next progress update likely next weekend.
Any insights welcomed!
One can get Pole Saw Extension Pole, at Sears Part #: 622892-00. This gives you the "trimmer plus" connection. I wonder if 3-D printing is strong enough for the inner shaft - square on one end, splined on the other? I have sheared the drive shaft on one pole pruner head - so strength of the square connector doesn't seem to the issue folks think it might be. Shaft sheared, the square part didn't round the mating end.
Side notes: I can even use the "trimmer plus" with a Stihl Kombi.
For snow clearing I have several of the discontinued snow paddles from Ryobi. With an eGo winter starting should be less of an issue - why one of our power heads has the external electric starter unit.
Background: One of them pesky Champions made a comment about ratios, so here's the investigation and report.
Test Units: 1) Ego PH1400/STA1500, and 2) Kobalt 80v Trimmer
Measurement Method: spin the shaft and count the trimmer head rotations (duh!), but I did sping the head and count the shaft to double check.
1) The Ego drive ratio is: 1.19 to 1, so if you spin the motor at 1000 rpm, it will 840 rpm; or if you spin the motor at 5000 rpm, the trimmer head spins at 4200 rpm.
2) The Kobalt drive ratio is: 1.36 to 1, so if you spin it the motor at 5000 rpm, the trimmer 3700 rpm.
3) Comparing the trimmers, the Kobalt spins 12.5% (1.19 / 1.36) slower, which means it has 12.5% more ratio advantage. If driving a heavy torque application like the chain saw pole attachment.. . well you'd have to measure those ratios and compare, so there is a difference, but more data is needed for a true conclusion.
Conclusion: The Ego drive ratio is close enough to the Universal Trimmer attachment ratio (12.5% difference) that an adapter to fit Universal Trimmer Attachments to the Ego PH1400 should be within the operational speed range of both the attachment and the PH1400.
PROTOTYPE adapter successfully connected universal trimmer attachments to the PH1400 Power Head
Cutting the splines. I set out to use nylon 6/6 bolts, but oddly those were more difficult to turn down to the initial diameter than soft metal (brass). so after spending 30 minutes with little success, I switch over to BRASS bolts.
I decided to use a 5/16" threaded rod as the primary shaft material. A 5/16-18 coupler was used so that different spine shaft material could be tried (which worked out really well after the nylon 6/6 issues.
Cutting splines by had was a real fun task. As such, the shaft is not completely centered, and when it starts to spin at low speed, it wobbles (see video below); however, once up to maximum speed (#2, high), it is complete vibration free. You'll notice in the video that right after spinning up, and right before stopping, the adapter will wobble. I will add a sealed bearing to the universal female square drive end, since the Ego already has a sealed bearing right behind the female spline drive (actually that is an assumption based on the 31" pole extension design). If the wobble is left unchecked, the female square drive adapter was rubbing on PVC pipe internal (starting to cut a groove, last picture).
With the brush cutter, that head has a lot of rotational inertia; and when you first spin it up, it torques noticeably. The work around is to spin up the motor slowly, which is very easy to do with the variable speed trigger.
Universal trimmer clutch, which is used for the female square drive, $20
brass bolts, 5/16-18 x 3-1/2", $3
coupler nuts, 5/16-18 x 7/8", $3
PVC tube (or aluminum), 1" OD x 6", $2
rod, threaded, 5/16-18 x 3" (1 ft min), $4
nuts, self-locking, 5/16-18, $2
universal trimmer coupler, $20 (estimate)
Build cost discussion:
The most expensive part is the universal coupler for $20. And the next most expensive part is the female square drive coupler, which would have been easier to fabricate from a 1/2" steel bar stock, than hand cutting the splines in the brass rod. Parts for a production unit would probably be around $35. So an adapter could easily be made/offered for $50-75.
Next PROTOTYPE changes:
1) add a bearing to support the adapter
2) switch the PVC tube to metal (likely aluminum)
My only concern with the design is whether the brass splines will stand up to a hard impact when using the brush cutter, so that is on my list of things to test once I get the sealed bearing support added. Should that fail, the solution would be to change from brass to steel, as that is the same material Ego is using for their production version; however, I do not anticipate that the brass spline drive will fail.
Ego Team, Suggestion:
As this prototype demonstrates, universal attachments appear to be very usable for use on the PH1400 power head. Please consider having your Engineering Team evaluate the technical feasibility, as well as the Marketing Team determine whether there is a market case which supports development.
Brush Cutter Test Satisfactory! (with minor adapter issue)
On the hills in my backyard, there is an undesirable plant that has large leaves and thick stalks. No string trimmer will effectively cut it (I tried for a couple of years with gas too); the trimmer string just wraps around the stalk, gets tangled and stops. I bought the brush cutter specifically for clearing these plants.
Test Setup & Attachment Details:
Ryobi RYBRC77, 8 in. reversible heavy-duty steel TRI-ARC blade. 
Battery: 2.0 AH and 2.5 AH
Within the first few minutes, I hit a soft log and it stopped the blade instantly, and also twisted the attachment body at the connection point (see pic below, 90 degrees out). Which wasn't a problem as I loosed the connection point, rotated it back into position, and continued on. And in a way, that is better than having something shear off (like the spline drive!).
One other thing I did try and test was the variable speed of the PH1400. In order to save battery life, I usually don't run it full throttle unless I'm going after a thick cluster of stalks. The speed modulation was pretty good. Just like my Kobalt 80v attachment capable trimmer, the Ego does take a second or two to get to full speed, but once you're at speed, it will hold it there without issue. Overall, I'm very happy with the way the Ego modulates the speed of the motor.
For this test, I cut an area about 20' x 30' (600 sq. ft), and these plants are spaced every 12"-18". The plants in the foreground are about 12" high, and that was a few months of growth; the uncut plants in the top half of the picture are about 3-4' high. These plants have a root bulb that sits just below the surface, and if you can cut down into that, the regrowth takes longer, which is what makes the brush cutter attachment so effective and controlling this plant. And since you're cutting so close to the ground, rock strikes are very common.
As the height and density of the plants got higher and heavier, the brush cutters started to get tangled up more. At one point, I snagged a 1/2" vine and it wrapped the blade and completely stopped the cutter. Most times I can power through the snag, and I tried this time, but the motor spun without the head moving; which made me think for a split second that Ego added a slipper safety clutch. But upon disassembling the adapter to check, I found that the spline drive adapter that I epoxied to the hex nut had broken the epoxy, and that joint was spinning freely (first picture below). I also inspected the brass spline of the adapter and was pleasantly surprised that the splines were holding up nicely; so at least in the short term, the brass is sufficiently strong (the red color is chicken blood made as an offering to the engineering gods). =)
My first idea was to hammer the adapter into a hexagon shape, but after the first hammer blow it was apparent that this was a hardened piece; and with the second blow the body developed some hairline cracks. At that point I moved to plan "B" which was to just hammer it enough so that the hex nut would not spin freely. I secured the hex nut with epoxy (same as last time). I performed an initial test of just running it under no-load in the garage and it works again as before.
The brush cutter head should be the most challenging attachment in terms of torque as well as impact tests. While the tiller would be a more continuous torque load, it also has a 72:1 gear reduction, so it has a huge mechanical advantage.
I'm going to use the chainsaw attachment this weekend on a tree pruning project, report to follow. I think this will work without issue due to the low load and low speed.
The 2000W motor of the PH1400 has sufficient power to drive a brush cutter attachment. I cannot wait until Ego releases the brush cutter blade for use on the STA1500 for the US market!
 Ryobi Brush Cutter on Home Depot:
Equipment in this post:
PH1400 power head
EP7500 31" extension
Troy-Bilt ~2-1/2' extension
Troy-Bilt Chainsaw attachment
First setup is 8-1/2 feet, second one is 11 feet. Of course the really long setup is only useful in a nearly vertical usage, as the poles tend to sag quite a bit.
NOTE: Ego's official position is that only a single EP7500 31" extension should be used, no more!
Build: v1.1, wobble fixed, bushing implemented, unit is now self contained
Summary: Simple bushing and alignment issue resolved
Find the wobble source
After thinking about it more, I disassembled the adapter and put only the brass rod spline in the PH1400, and when spinning it had about 1/16" of run-out, so I bent it to minimize the run-out.
Compounded Error /!\
Then I added the 5/16" coupler back, as well as the steel connector rod, and the final female square drive adapter. The run out caused by the coupler was pretty significant. I was assembling it by butting both the brass threaded rod and the steel threaded rods together. However, after thinking about it, unless those faces are absolutely flat and sit flush, that could cause the rods to be out of concentricity, and would therefore introduce a wobble at the female square drive end. The wobble would be on the female drive side because the brass spline end is supported by a bearing right next to the female Ego spline drive.
The fix was very simple, when making up the coupler joint, I left a small gap between the two rod faces and that was enough to have nearly zero run-out. Which left the issue of supporting the spinning rod. Going back to my temporary fix of adding a nylon washer was a non-starter. I demonstrated this for myself by spinning the adapter and accidentally touching the nylon washer, which was spinning so fast that it gave me a friction burn from a split second of contact. Between the wobble from the misaligned coupler, and the high speed spinning washer, it was no wonder the inside face of the PVC pipe was being chewed up. It would have been only a matter of a few more runs before that cut completely through the PVC tube.
KISS (Keep It Slippery Stupid)
So off to the hardware store again, and this time I'm looking for a very specific sealed bearing, a 698Z, which is 8mm ID, 19mm OD, and 6mm WIDE. I had low hopes of finding one, so I looked for alternatives. If you look back at the original square drive, those flexible shafts are riding inside of a very long and smooth tube. I found a "plastic socket" which is for mounting casters inside of metal tube (chair) legs. The plastic is a very hard type which is perfect for this application. In short, this is the perfect piece to keep the adapter spinning dead center. Another reason this works well (and why the nylon washer does not) comes down to surface area. The 5/16" diameter and 1" long "bearing surface" of the new plastic bushing is much larger than the 3/4" diameter by 1/8" wide nylon washer, by 675%. 
For good measure, I packed the bushing and threaded rod with white lithium grease.
As I posted above, the nylon washer "temp fix" was a terrible idea. That was a band aid fix (which wouldn't have worked long term) for the real problem which was a summation of many little misalignments, which compounded into a really bad wobble. The videos of the PH1400 under no load is very promising, especially when at "HI" speed.
Going to put some run time on this setup and see how the bushing holds up. If the threaded rod chews up the bushing too much, the next thing will probably be to jump to a sealed bearing solution.
 comparison of bearing surfaces: 5/16" x 1" = (2*pi*r or pi*D, 1.99 in^2), vs. 3/4" x 1/8" = pi*D, 0.29 in^2). The long bushing has 6.75 times more surface area (675%)!
Discussion: No compatibility issues identified with either trimmer head.
Conclusion: Going up and down the steep hill, the 15" dedicated trimmer (ST1500) is a more appropriate tool for the job, in both weight and battery endurance (I can usually do the entire hill (Up/Down) on a single 2.5 AH.
I know the EGO power head is not compatible with the "standard" square shaft used by several other brands.
It seems almost every brand uses that square shaft, but I think there are maybe a couple that don't. Does anyone know if any of those happen to fit with the EGO unit?
I'm thinking of Honda's gas powerhead specifically. I believe Chervon is making Honda's new European line of battery powered tools, right? So there's already some cross-licensing or sharing of products between these brands. Maybe their attachments would fit on the EGO? It looks like they use a splined shaft, but from what I've seen can't really tell if the number of splines or size matches.
I've also seen a brand called EcoPro that might have a splined shaft, anyone know anything about that?
I'd really prefer to buy Ego attachments since I've been impressed with their other tools so far, but with the availability issues, and attachments like the hedge trimmer that are not even offered yet, this could help fill some gaps.