Front mount flail mower from mid PTO

[Paul Allwood]

I'm still playing with ideas for a front driveshaft, but one that provides greater movement than can be achieved with just the mid-PTO shaft and the female spline sliding over it.

For now I'm going to just make a 3-point mounted finish mower. Might try something different like a front mounted flail mower sometime later.

I'm thinking of a rear side-shift finish mower, partly to avoid the wide arc when turning, partly to provide a bit more flexibility, partly for something just a little different.

Don't you hate it when you think you've had a new idea and then a Google search tells you it's already done ? Turns out Fischer Australis has one that's made in Italy, and it can be driven from the front or rear....it looks like this:



Maybe there are others ???

 

[Paul Allwood]

I was just flicking through the specs in the operator's manual for my B7200 and came across something that seems strange....probably because I don't understand it.

In the implements section of the manual it includes maximum widths and weights for a 2 or 3 blade rotary mower - I'm guessing this must be what is normally called a finish mower.

Maximum widths listed are as follows:
- Mid mount: 60"
- Rear mount: 48"

The maximum weight in each case is 140 kg.

I don't understand the reason for the different width ratings between mid and rear mount. Does anyone know why this might be ? Is there a different power rating for the mid vs rear PTO ?

Here's the relevant extract from the manual:

 

[TheOldHokie]

I was just flicking through the specs in the operator's manual for my B7200 and came across something that seems strange....probably because I don't understand it.

In the implements section of the manual it includes maximum widths and weights for a 2 or 3 blade rotary mower - I'm guessing this must be what is normally called a finish mower.

Maximum widths listed are as follows:
- Mid mount: 60"
- Rear mount: 48"

The maximum weight in each case is 140 kg.

I don't understand the reason for the different width ratings between mid and rear mount. Does anyone know why this might be ? Is there a different power rating for the mid vs rear PTO ?

Here's the relevant extract from the manual:

View attachment 177205

They are two different PTOs with different speeds and HP ratings. In real life my B7200DT had no trouble spinning a 72" rear finish mower. Weight was a different matter

Dan

 

[Paul Allwood]

They are two different PTOs with different speeds and HP ratings. In real life my B7200DT had no trouble spinning a 72" rear finish mower. Weight was a different matter

Dan

Thanks Dan.

My reason for asking is that one thing I've been considering was building a front mounted flail mower.

I've been sick the last couple of weeks so no progress except thinking. Now I'm back to thinking a flail mower on the front (run from the mid PTO) might be better than the rear.

Not thinking of doing it, but would it mean a front PTO running off the crankshaft would be rated at engine power, ie, 17 HP ?

 

[Paul Allwood]

I've been crook for the last couple or three weeks so have only just started getting back out to the shed. I'm back to thinking I'll have a go at a front mounted flail mower. If you're thinking that I find it hard to make up my mind and make a decision you'd be pretty right.

I made a new drive shaft to go from the mid PTO to the bearing block I've mounted on the front diff housing - this must be version 764. For this one I used a piece of square tube inside a piece of bigger square tube. I had to grind a little off the smaller tube so it ended up a pretty neat fit. The tubes overlap about 80% of their length so there's almost no sideways play.

New uni joints arrived today so I fitted them (I'm 63 years old and these are the first ever uni joints I've fitted), assembled the shaft and bolted it in place for a test run.

It seems to work ok with not enough run out to bother me.

Here it is running at about 600 rpm (idle and low PTO speed selected).

Here it is running at about 2500 rpm (engine 2500 rpm and high PTO speed selected). This is what it will run at to power the flail mower.

I also had to take a bit of of the inside tube to clear the internal weld bead of the outer tube.



I think it may have been @Russell King who pointed out that you can buy tubular sections intended to slide one inside the other for PTO shafts. I had already made this square tube version by the time I had seen that, otherwise what Russell suggested would have been far easier.

Next I'll take it off, pull it apart, paint the bits that need painting and grease the rest. Also need to make some guards to keep long grass and fingers out of the spinny bits.

 

[PoTreeBoy]

I've been crook for the last couple or three weeks so have only just started getting back out to the shed. I'm back to thinking I'll have a go at a front mounted flail mower. If you're thinking that I find it hard to make up my mind and make a decision you'd be pretty right.

I made a new drive shaft to go from the mid PTO to the bearing block I've mounted on the front diff housing - this must be version 764. For this one I used a piece of square tube inside a piece of bigger square tube. I had to grind a little off the smaller tube so it ended up a pretty neat fit. The tubes overlap about 80% of their length so there's almost no sideways play.

New uni joints arrived today so I fitted them (I'm 63 years old and these are the first ever uni joints I've fitted), assembled the shaft and bolted it in place for a test run.

It seems to work ok with not enough run out to bother me.

Here it is running at about 600 rpm (idle and low PTO speed selected).

Here it is running at about 2500 rpm (engine 2500 rpm and high PTO speed selected). This is what it will run at to power the flail mower.

View attachment 177267

View attachment 177268

View attachment 177269

View attachment 177270

I also had to take a bit of of the inside tube to clear the internal weld bead of the outer tube.

View attachment 177271

I think it may have been @Russell King who pointed out that you can buy tubular sections intended to slide one inside the other for PTO shafts. I had already made this square tube version by the time I had seen that, otherwise what Russell suggested would have been far easier.

Next I'll take it off, pull it apart, paint the bits that need painting and grease the rest. Also need to make some guards to keep long grass and fingers out of the spinny bits.

That's some fine work!

 

[Paul Allwood]

That's some fine work!

Thanks. I'm keen to see if it survives if I can manage to get a mower connected to it.

 

[Paul Allwood]

I've finally thrown my hat over the fence and started making a flail mower.

The first bit I've made is the rear roller using 100 mm NB medium wall pipe (approx 4.5" OD, 4.3 mm wall). I settled on a length of 1400 mm - longer than would normally be specified for my 14 HP PTO, but I'll be using Y blades and only mowing grass. If that turns out to be too long I'll guess I'll have to drive slower and enjoy the scenery more, or cut it shorter.

Here are the axles for each end of the roller pipe - I copied this design from a video by Greenhill Forge.

Slow progress on a small lathe.

One for each end.



Tap tap tap....







Weld both ends.



Grind the rough welds....still looks rough. I'm definitely not a welder but I don't reckon it will break.



Put a bearing on each end and test runout - doesn't really matter for the roller but I wanted to see how close it was.

https://youtube.com/shorts/1QrJw-3ygcE?si=QN_lfO-u8NXe0Kl8

About 0.75 mm runout at the centre - plenty good enough for a roller. When I make the cutter drum I'll run the shaft all the way through to give me the best chance of getting the lowest runout.

QUESTION - is PTO overrun protection important for a flail mower ? If so, I'll need to buy a suitable adaptor as it's not a feature of the mid PTO on my tractor (only the rear PTO has it).

 

[barts]

QUESTION - is PTO overrun protection important for a flail mower ? If so, I'll need to buy a suitable adaptor as it's not a feature of the mid PTO on my tractor (only the rear PTO has it).

This really depends on the effective inertia of the flail mower. My B5100 doesn't have an overrunning clutch on the rear PTO, and it's rather exciting to use with a brush hog. Of course, a flail mower has less rotating mass, but as you're spinning it pretty fast I'd suggest using one on that front spline; they're easy enough to add (just means a bit shorter drive shaft to the mower). You can always wait and try it out to see if it's needed; if the tractor keeps going when you push the clutch in you'll know !

 

[North Idaho Wolfman]

One thing you could do is drill a spot weld the back disk to the roller.
Looking great!

 

[Paul Allwood]

One thing you could do is drill a spot weld the back disk to the roller.
Looking great!

Thanks. Good idea - I think I'll add three spots spaced equally around the circumference.

 

[North Idaho Wolfman]

Thanks. Good idea - I think I'll add three spots spaced equally around the circumference.

Oh and in case you missed the other comment.
You do not need a overrun clutch on a mid PTO HST tractor.
In fact an over running clutch on 2500 RPM would be extremely dangerous!

 

[Paul Allwood]

Oh and in case you missed the other comment.
You do not need a overrun clutch on a mid PTO HST tractor.
In fact an over running clutch on 2500 RPM would be extremely dangerous!

Thanks, I did see that. Is that because the PTO disengages when the clutch is depressed ? Just out of interest, why would it be dangerous ?

 

[Paul Allwood]

In answer to a query by @Bee-Positive regarding flail mower rpm....

I've looked at the specs of lots of flail mowers and the upper end of blade tip speed is around 50 m/s for mowers around three size in making. I'll start by aiming at this higher speed as I'm thinking the higher speed will give a better cut.

Here are my calculations to get to that using:

- The old finish mower gearbox which gives a 1:1.187 speed increase,

- A 2B-4" to 2B-4.5" pulley which gives a 1:0.889 speed decrease,

- Mid PTO: 2500 rpm

Drum speed = 2500x1.187x0.889 = 2638 rpm

- Blade tip diameter of 362 mm

Blade tip speed = 0.362 x Pi x 2638/60 = 50 m/s

If I need to adjust this once I get it running I'll be able to change one or both of the pulleys.

 

[North Idaho Wolfman]

2500RPM is actually what you want them to run at!

 

[Paul Allwood]

One thing you could do is drill a spot weld the back disk to the roller.
Looking great!

Done - 3 spots at each end. Thanks for the suggestion.

 

[Paul Allwood]

A little more milling on the lathe this afternoon to cut a couple of keyways in shafts - both will be used with taper-lock bushes in pulleys. One pulley will be on the end of the shaft from the gearbox. The other pulley will be the drive pulley on the cutter drum shaft.

It's a slow process, but it works.

27 July 2026

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27 July 2026

Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube.

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Something like this isn't really a big deal for the real machinists out there, but I'm far from a real machinist and these are the first keyways I've ever milled - pretty happy with how they turned out.

The bloke at the bearing supply shop suggested that I don't really need to use a key with a taper-lock bush, but I don't think adding a key will do any harm.

 

[Paul Allwood]

This afternoon I got the shaft and end caps welded into the cutter drum pipe. I think I did a better job of the welds today.

This is the drive end - the shaft will go through a bearing and end with a 2B pulley. The other end just gets a bearing.



Here's the whole bare cutter drum.



It runs pretty true.

30 July 2026

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It seems like ridiculous overkill to put a dial indicator on a bit of black pipe, but I was keen to see how true it was running, and had to measure it with something. Highest to lowest is about 0.3 mm (12 thou). It looks mostly consistent around the circumference, about 0.3mm low close to the weld seam. I'll take it.

30 July 2026

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Next step is to make and weld on the tabs that will hold the flail blades onto the cutter drum.

 

[Paul Allwood]

I spent the afternoon today making the tabs that will be welded to the cutter drum to mount the flail blades.

I used the plasma cutter to cut a radius to match the cutter drum pipe.

There was a bit of clean up required on the linishing belt, then off to the drill press using a simple jig to locate and drill the bolt holes. Centre drill first followed by 12.5 mm to suit M12 bolts.

I need 72 of these tabs to mount the 72 flail Y-blades.

 

[North Idaho Wolfman]

I'll throw this out there just in case you haven't considered it.

When you put the tabs on the roller for the flail blades mount them in a helix pattern.
It will self balance that way.
And it takes less hp to power them.