Garden Railway Forum

I recently came across these very small servo's - 17 x 6 x 19mm and at 8 quid each they aren't expensive. May not be man enough for hefty steam loco's but could be useful in other circumstances.

https://www.ebay.co.uk/itm/175319678755

These popped up beneath it
https://www.ebay.co.uk/itm/352960380402

They suggest they are equally as strong as other servos. Not sure how their torque compares.
They measure 17.5×20×8.3mm so only slightly larger.

Rik

ge_rik wrote: ↑Wed Sep 14, 2022 7:53 am These popped up beneath it
https://www.ebay.co.uk/itm/352960380402

They suggest they are equally as strong as other servos. Not sure how their torque compares.
They measure 17.5×20×8.3mm so only slightly larger.

Rik

At least they have the advantage of the standard servo plug.

I had a particular potential application in mind when I went looking and the smaller the better in my case. I also have some of the micro JST's, so that isn't a problem.

I will have a look at these.

I am thinking about servos for controlling the valve gear on Little wonder. There isn't a lot of room, on an inside framed loco to fit on 32 mm track!

The first one you found quotes a torque of 0.075Kg.cm
The second one quotes 800grams - which isn't a torque. Presumably they mean 0.8Kg.cm - but it might be 0.080kg.cm (which I suspect is more likely because it is close to the value for the first one) However if it were 0.8Kg.cm - then it would be worth £16 to try it!

As a comparison the one Micron recommend for steam locos quotes a torque of 1.6Kg.cm - it is just too big for what I am looking for.

So assuming that these values found on websites are all correct these sub miniature servos really have little ability to even overcome friction.

I did look at linear servos as these seem small. Any other ideas?


Trevor

One other thing to look out for, with the newer micro-servo's and actuator's..
Many are designed to work with a '3.5V' supply, as they are designed for use with 3.7V '1S' Lithium batteries.

It should be in the specifications, but can be lost in the Chinglish.

Phil.P

Phil.P wrote: ↑Fri Sep 16, 2022 11:57 am One other thing to look out for, with the newer micro-servo's and actuator's..
Many are designed to work with a '3.5V' supply, as they are designed for use with 3.7V '1S' Lithium batteries.

This is a real advantage! The servos I have used required 4.8v so I needed to find space to stick 4 AAAs - not necessarily easy on a small loco...

SimonWood wrote: ↑Fri Sep 16, 2022 12:02 pm

Phil.P wrote: ↑Fri Sep 16, 2022 11:57 am One other thing to look out for, with the newer micro-servo's and actuator's..
Many are designed to work with a '3.5V' supply, as they are designed for use with 3.7V '1S' Lithium batteries.

This is a real advantage! The servos I have used required 4.8v so I needed to find space to stick 4 AAAs - not necessarily easy on a small loco...

Simon,
Many, with the smaller connector, will happily work from a '1S' Lithium battery. - Trying to drive them from three NiMH, does not tend to work, as they have a steeper slope as they discharge.
The Lithium batteries tend to hold-up a lot better, in this respect.

Whether your receiver will 'like' 4.8V might be something else to consider. - Though those designed to drive servo's, tend to be fine.

Phil.P

I have checked with the supplier of the slightly larger one ( the second one you found.)

The torque is 0.7kg.cm at 4.8 volts. Also it will work at 4.8 volts. So while it isn't as powerful as the one I mentioned for steam use - it is half as powerful - which I think is significant. I will certainly get a couple to try them out.

Trevor

Phil.P wrote: ↑Fri Sep 16, 2022 12:15 pm Simon,
Many, with the smaller connector, will happily work from a '1S' Lithium battery. - Trying to drive them from three NiMH, does not tend to work, as they have a steeper slope as they discharge.
The Lithium batteries tend to hold-up a lot better, in this respect.

And they are smaller!

I think the receivers would be happy too. I use Deleting Rx102s (3.2V+) though I haven't tried a MR001 (3.45V+) yet...

The quoted dimensions and the performance of these PES GH-S37D servos seems to vary from website to website, but the specs on this link seem typical. Torque is quoted as 0.7kg/cm at 4.8V, but they have plastic gears with very fine teeth and it should be pointed out that polymers rapidly lose strength as the temperature increases.

https://www.aliexpress.com/item/629392186.html

I have Hitec HS-65MG servos with metal gears in the Garratt and I'm running them on 4.8V, so the torque will be about 1.6kg/cm in practice. One servo will move two Roundhouse slide valves under steam with no strain, but I've never bothered measuring the actual force required with steam on the valves. I do know that even with no steam on the valves it takes about 0.3kg/cm of torque on the weighshaft just to change direction with Roundhouse valve gear.

Regards,
Graeme