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Thread: Another one bites the dust...servo

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Old 02-14-2019, 02:53 PM   #41
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Default Re: Another one bites the dust...servo

Quote:
Originally Posted by jbeech View Post
The Savox is a good servo. So is the Holmes. Mine isn't bad either. One thing I believe you guys should all consider is using a sacrificial part.

If possible, I recommend folks run our heavy duty plastic servo arms (some rigs like an X-MAXX require a special servo arm). Anyway, they're only a buck each, but a bag of 25 gets the price down to 68 (and splitting a bag of 100 with some pals gets it down to 50 each). The idea is, when it breaks (not if), you're only out the few minutes it takes to replace it (plus some small change).

The alternative is being out of contention (if you're competing), plus the cost of the damaged or destroyed servo (mine or my competitors). Then there's the time required to remove and replace the servo, which is longer than it takes to R&R an arm. And don't forget the time waiting on repair/replacement if you don't have one on hand (plus postage costs).

Anyway, I'll never get rich off these servo arms - but they can definitely save you some dough. By the way, in mechanical engineering the concept of sacrificial parts is at least as old as the concept of the fuse in electronics. Applying it to your rig can save you more than money because a lot of you can only get out to the trails occasionally and spoiling this time because of a broken servo (which can happen just minutes into the run) absolutely stinks!

ProModeler single-horn HD servo arm: https://www.promodeler.com/PDRS101
It's a great idea, except that they would strip constantly. It's not uncommon to strip inexpensive aluminum servo arms when rock crawling. I'm not sure that any plastic servo arm could withstand 500oz-in of torque.
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Old 02-14-2019, 05:12 PM   #42
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Default Another one bites the dust...servo

Quote:
Originally Posted by jbeech View Post
The Savox is a good servo. So is the Holmes. Mine isn't bad either. One thing I believe you guys should all consider is using a sacrificial part.



If possible, I recommend folks run our heavy duty plastic servo arms (some rigs like an X-MAXX require a special servo arm). Anyway, they're only a buck each, but a bag of 25 gets the price down to 68 (and splitting a bag of 100 with some pals gets it down to 50 each). The idea is, when it breaks (not if), you're only out the few minutes it takes to replace it (plus some small change).



The alternative is being out of contention (if you're competing), plus the cost of the damaged or destroyed servo (mine or my competitors). Then there's the time required to remove and replace the servo, which is longer than it takes to R&R an arm. And don't forget the time waiting on repair/replacement if you don't have one on hand (plus postage costs).



Anyway, I'll never get rich off these servo arms - but they can definitely save you some dough. By the way, in mechanical engineering the concept of sacrificial parts is at least as old as the concept of the fuse in electronics. Applying it to your rig can save you more than money because a lot of you can only get out to the trails occasionally and spoiling this time because of a broken servo (which can happen just minutes into the run) absolutely stinks!



ProModeler single-horn HD servo arm: https://www.promodeler.com/PDRS101


I have always wondered about doing this. On my setup today, everything is rock solid, and I have no idea which expensive part will fail on the next big crash. This would give me a known failure point, which would be cheap to replace and easy to access. As long as it lasts under normal conditions, ie not crashing...


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Last edited by w0pgk; 02-14-2019 at 05:14 PM.
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Old 02-14-2019, 05:56 PM   #43
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Default Re: Another one bites the dust...servo

I crawl and creep. Wobble introduced by the servo arm flexing is not an option. I need to point and roll into the line, that 1/4 off line might be the difference. That all said, John do your plastic arms flex?


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Old 02-14-2019, 07:39 PM   #44
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Default Re: Another one bites the dust...servo

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Originally Posted by jbeech View Post
The alternative is being out of contention (if you're competing), plus the cost of the damaged or destroyed servo (mine or my competitors). Then there's the time required to remove and replace the servo, which is longer than it takes to R&R an arm.
But, if youre competing there is no sacrifice. Youve already lost if you gotta replace your arm anyway. Gotta take that chance. You get your cake and eat it too when comping, especially in servos You need to be able to trust your gear with something strong, doesnt stall and wont break. Sure, everything has a failure point but why cut yourself short after you build a $1000-$1500 comp rig for its durability and abilities?
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Old 02-22-2019, 03:46 PM   #45
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Default Re: Another one bites the dust...servo

My idea of a fuse in the steering is the Revo rod ends. I've broken a bunch of them without damaging my promodeler servos.

Running a plastic horn just isn't an option for me. I'm so hard on equipment that plastic horns would literally last minutes.

I'm still waiting for the 420v2 in my bomber to die. Its over 3 years old now and has outlasted literally every other part on the front axle. Meanwhile my collection of 470's has almost grown into double digits as other brands have died.
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Old 02-23-2019, 07:50 AM   #46
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Default Re: Another one bites the dust...servo

So I have to ask after reading through this thread, IF you run a bullet proof servo off a 6s lipo (sarcasm intended), strong enough to use as a jack for your daily driver, AND the servo horn is up to the same capabilities, AND the rod ends don't give way, what did you bend or break in their place? Did you rip the servo off the axle housing or bend links? Did you rip the lockout off the axle? Something catastrophic is going to happen. In the real world (1:1), a fusible link is engineered into the system, be it a cheap or expensive replacement and ideally a quick change out during a race. Is this not a consideration in the RC world? My rig is different than all the rest, but has a 380 in./oz with a servo saver that was going to get swapped for a big block servo, but I'm having second thoughts. When/if I do, that plastic servo arm mentioned above will go on as well. I like cheap insurance, and yes I also firmly believe it is a fact of when, not if.
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Old 02-23-2019, 01:32 PM   #47
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Default Re: Another one bites the dust...servo

For me, the next failure point was the steering arm on top of my knuckle, I have a lot of leverage on it though, having the tie rod and draglink stacked on top of it. It didn't have a catastrophic failure, but it bent upwards while my 700oz servo was trying to push it right and the 2 pieces of granite that the car was wedged in said "NO!" I just noticed a little extra toe, and a slight left hand pull afterward. I'll probably move my weak link around again by making that steering arm out of 3/16" next time, instead of the current 1/8". The rod ends on my draglink get sloppy enough to want to change them after a month or so, switching to RPM's didn't seem to extend those service intervals much, so I'm trying a more solid setup now. I also wear out the top right brass knuckle bushing at an accelerated rate, but again, it just slowly gets sloppier until I am bothered enough to change it. Still cheaper than throwing a bunch of cheap servos at it every month or 2.

I'm all for a fusible link, but at this point, I'd only want it to protect the servo, being the most expensive part of the system by far. Once I've moved the weak link to something like the stub shaft, knuckle, or c-hub, then I'll say its gone too far. I'd have to hope that if a servo puts out X amount of torque, their gearsets could withstand X+some before failure, but we know that's not always the case. Imagine that your servo puts out, lets say 470oz of torque, and the gearset will put up with a make-believe 550oz, so you have a link that will fail at 500oz and everyone is happy, right? That is, until Servo McTailswinger shows up with 600oz+ and the fuse is letting go every 5 minutes. I'd sooner put up with a weak servo that just stalls, but lives to crawl another day, than deal with the previous scenario.

I chose to go to hydraulic steering instead, now instead of stripping gears, my pump just bleeds off excess pressure

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