Question about servo torque

http://www.ebay.com.au/itm/380704435503?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

I believe the servos that this hexapod chassis is designed for are roughly 13kg/cm ones.

Does this seem like overkill to anyone else?

Where a talking about each leg being able to support well over 1kg depending on their precise lengths.....which is a bit hard to tell exactly from the photos.

Unless this chassis is quite a bit larger than what I am imagining.

I was planning to make my own similar chassis out of 3mm PVC, which is easier to work with than Al and costs nothing since you can make flat PVC sheets out of large PVC pipe if you slit them down their length and put the PVC in the oven on 200 degrees or so for several minutes.

I figure that if the main chassis is roughly 12cm wide by about 20cm long, with appropriate cut outs or holes to reduce the overall weight, and the leg segments are 5-8cm long then servos of 2.5kg/cm torque would probably do the job.

That would mean each leg could support around 400g or so, times a minimum of 3 legs on the ground at a time, which would be over a 1kg.

7.2V NiMH or LIPO 2400mAH battery back is only going to weigh 200g or so.

Thoughts?

Just for the record, torque is kg * cm not kg / cm

(In fact that's not strictly true either, since kg isn't a measure of force, but wth...)

Most commercial designs use/recommend servos of a size the is known to work acceptably well. Below is a hexapod project that is using Tower Pro SG5010 servos. Bottom is a hexapod project that uses wood instead of plastic.

Its NOT overkill.
13kg*cm == 127.5Ncm == 1.275 Nm. Thats about the torque required for a M3 screw. And consider that's the stalling torque, the servo will not be able to move when loaded that much. In order to get both speed and precision you must have some safety margin here.
And do not confuse torque with force. Force(N) == Torque(Nm)/TorqueArmLength(m) perpendicular to the torque arm.

JimboZA:
Just for the record, torque is kg * cm not kg / cm

(In fact that's not strictly true either, since kg isn't a measure of force, but wth...)

Joules per radian or newton-metres :slight_smile:

Actually whenever you have a mechanical spec like torque you always add a generous
safety factor, so if the servo is 13 kgf-cm (1.3 N-m), then only expect half that - this means
the motors won't be struggling, its still going to work when the batteries are low and
everything lasts longer (mechanical things wear out fast when pushed hard).

[ and of course some specs are more marketing fantasy than real-world measurements ]

You should learn what torque is.