7955 Woes - concerns of a relative nugget

[Pushjerk]

What up, RCC,

Dropped an HS7955TG in my Wraith recently after trashing the stock Tactic servo during a light bashing session. Was only a matter of time. In fact, I'm surprised it lasted as long as it did. But we're not here to discuss high failure stock electronics - let's talk instead higher end products and our expectations of them.

Prior to installation, and upon initial power-up to center the servo, I was immediately a bit surprised to hear the "whine" and "chatter" coming from the servo. A quick look on the forums revealed this is pretty normal for this piece of gear. The Hitec guys call in the "sound of precision," I've read. Though, I was more unimpressed to see the slightest bit of jitter at the spline of the servo - this occurred shortly after power up and seemed to subside after it was powered for a few. Let's install and run it. Finally, prior to running, of concern to me was how freely the steering assembly moved when the system is powered off - all, and even the cheapest of servos that I have used throughout my time in RC, and especially those in the standard category, have displayed a fair bit of resistance to movement when powered off, but the 7955 swings quite freely, almost as if the drag link is unattached (a slight exaggeration). Normal?

Now to servo operation. When powered on it is a beast. And man does it "moan" when it works. I have, though, been pretty unimpressed with the centering of the servo. Noticeable when on the trail during a straight-away, and quite when testing on the street, steering requires constant trim back and forth to keep straight. Frustrating.

As stated in the title, I am a relative noob to crawling and surface RC, though I've been flying fixed wing for years. I have always been impressed by Hitec's quality. I am, however, a tad underwhelmed at the performance of the 7955 - I expect plenty more for the price tag.

Curious to hear from all you 7955 owners and operators out there. Any similar experiences? Might I have an "exceptional" unit? Hitec's excellent custy service is only a phone call away, though thought I would hit the forums first to get the community's take on it.

Replies appreciated.

[Awesome-Sauce RC]

not a hitec servo user but are you powering the servo with the voltage provided to the rx via the esc or using a bec?

even if the servo is getting 6v from the esc it may not be getting the current it needs, i use a savox @6v with less torque than your hitec on my scx10, its fed via cc bec and has no issues centering under the load weight of the vehicle even with my 2s 5000mah lipo mounted up front directly above it.

that servo could potentially pull up to or more than 5A when it needs to, so if no bec then that may be your answer for the centering torque performance at least.

the noise unfortunately is inherrent to high power servos but maybe their are some quieter brushed/coreless servos available im not sure on that though, my savox sings away when its sitting idle maintaining its centre position as many others get with their servo upgrades, only way to get rid of that as far as i can tell is to go for a brushless servo, i picked up a cheap turnigy bls25hv from hobbyking for $30 and the only thing i hear from that is the geartrain but more power than my savox running at the same voltage, should make for an interesting servo winch lol.


tony

[Pushjerk]

Thanks for the reply, Tony. The truck is indeed running a stand alone BEC , running at 6V- Should've stated that in the long winded first post. As far as the noise, I could care less. Doesn't bug me. Though when you hear it for the first time and are not indoctrinated, comes as a surprise.

[novarider]

Mine centers fine. It may twitch slightly with nothing attached as it's searching for dead center. You should be able to sub trim it straight tho, if it constantly changed I would think something is wrong and exchange the servo for another new one.

[flatfoot]

I have 3 of them, all ran from a bec.all 3 chatter when at idle, but all center up fine.sounds like you might have an "exceptional" unit that needs attention.hope you find a solution.

[badhoopty]

using a servo programmer to adjust the deadband helps alot with chatter.

[The Violator]

Even the 7955 is slightly underpowered for a 2.2 truck. I would look next time at the 7950 or its newer equivalent. 400+ ounces at 7.4 volts.


I have about a dozen 7950's in crawlers and have never burned up one. Actually I have only burned up one Hitec servo ever, and I believe it was a short in the power wire causing the meltdown.

[84yoda]

7950 is almost 500oz. You should bump up your volts to 7.4v on the 7955, its almost 400oz.

[Pushjerk]

Thanks for the replies, gentlemen. I'll continue to play with the servo and determine if the centering troubles need addressing. Regardless, looks like I need to get my hands on a Castle Link and a Windows machine for some 7.4V action.

Though the 7950 is indeed a powerhouse, I think the 7955 is plenty of servo for my rather light mostly stock truck and my driving style - trailing with the kids, light crawling here and there, exclusively recreational. But I'll keep an eye on the classifieds for any 7950s that pop up.

[flatfoot]

I run all mine @ 7 volts.1st one was in a wraith, never had any problem turning those 2.2's.now all are in scx10's, still @ 7 volts.plenty of steam to turn them, without pushing them too hard.aside from being noisy, no issues.

[emilsolbat]

Are you sure it's not just slop in the steering links? If it's a stock Wraith (apart from the new servo), the plastic links are pretty flimsy.

[Pushjerk]

That would be a prime area of concern, though I've shown my steering assembly a bunch of tax return love lately with VP links, chubs and knuckles. Plenty of running and tuning following install - it's pretty darn solid, only bit of slop exists where I'm using the stock (brass?) bushings from the Axial knuckles in the VPs. Will post results and resolution as they develop. Again, guys, thanks for all the replies.

[JesterSpec]

Even with VP it will be difficult to get perfect center you'd expect in the air on a crawler. Not only the steering links, but the 4 link will let the rig wander (just like a 1:1)

[Eric0424]

Originally Posted by Pushjerk

That would be a prime area of concern, though I've shown my steering assembly a bunch of tax return love lately with VP links, chubs and knuckles. Plenty of running and tuning following install - it's pretty darn solid, only bit of slop exists where I'm using the stock (brass?) bushings from the Axial knuckles in the VPs. Will post results and resolution as they develop. Again, guys, thanks for all the replies.

The brass bushings are probably Vanquish or another aftermarket part, the stock Axial are black oxide coated steel.

Knuckle Bushings (4pcs) - Vanquish Products

Axial Racing - Flange Pipe 3x4.5x5.5 (4pcs)

[btoungette]

Originally Posted by Pushjerk

... Regardless, looks like I need to get my hands on a Castle Link and a Windows machine for some 7.4V action....

If you want 7.4 volts and a cheaper alternative, get the 7954. Basically the same servo with ability to run 7.4 volts. Hitec doesn't recommend 7.4v on the 7955.

Per Hitec website for the HS-7955TG:

Specifications

Motor Type:	Coreless
Bearing Type:	Dual Ball Bearing
Speed (4.8V/6.0V):	0.19 / 0.15
Torque oz./in. (4.8V/6.0V):	250 / 333
Torque kg./cm. (4.8V/6.0V):	18.0 / 24.0
Size in Inches:	1.57 x 0.78 x 1.45
Size in Millimeters:	39.88 x 19.81 x 36.83
Weight ounces:	2.29
Weight grams:	64.92

I run both the 7955 and the 7954. I wouldn't recommend the 7955. The 7954 just seems to work better (maybe the higher voltage capability). However, I did change some of the programming on my 7955 and the noise/chatter was reduced (didn't go completely away, just reduced).

[badhoopty]

are your steering arms tight on the knuckles? ive had those screws back out over time and it only takes a bit of play here and there to compound.

its also good to get fresh bearings and shim the slop out of the stub axles. nowadays with shimming this and that the majority of my steering slop is usually from the stub axle bearings.

also... this is complete speculation and it might just only prove im no good at adjusting them, but i think the grub screw style lockers that lock the axles down not only can create binding and wobble if not adjusted right but also tend to wear out knuckle bushings and stub axle bearings because it fights against the axleshafts ever so slightly being pulled out or pushed in depending on the angle of the steering. ive always had a hard time getting a grub locker style diff to not bind or drag the axleshafts at some point of steering travel. (especially after no slop knuckle bushings) but if i loosen up the screws just a tad so the axles have a bit of freedom to move in and out, i get much smoother steering and no binding on the axle shaft joints.

[JokersWild]

Originally Posted by btoungette

If you want 7.4 volts and a cheaper alternative, get the 7954. Basically the same servo with ability to run 7.4 volts. Hitec doesn't recommend 7.4v on the 7955.

Per Hitec website for the HS-7955TG:

Specifications

I run both the 7955 and the 7954. I wouldn't recommend the 7955. The 7954 just seems to work better (maybe the higher voltage capability). However, I did change some of the programming on my 7955 and the noise/chatter was reduced (didn't go completely away, just reduced).

7955 was built before high voltage servos were out . many plane guys been running them off of 2s lipo for quite some time. They will live

[84yoda]

Originally Posted by btoungette

If you want 7.4 volts and a cheaper alternative, get the 7954. Basically the same servo with ability to run 7.4 volts. Hitec doesn't recommend 7.4v on the 7955.

I run both the 7955 and the 7954. I wouldn't recommend the 7955. The 7954 just seems to work better (maybe the higher voltage capability). However, I did change some of the programming on my 7955 and the noise/chatter was reduced (didn't go completely away, just reduced).

You should do some homework before posting incorrect info.

There are PLENTY of sites online who run 7955 at 7.4v and have confirmed with hitec PLENTY of times that it is 100% ok to run 7.4v. They run same high voltage board inside. So 7955 over 7954 all day, titanium gears and there same price.

[btoungette]

Originally Posted by 84yoda

You should do some homework before posting incorrect info.

There are PLENTY of sites online who run 7955 at 7.4v and have confirmed with hitec PLENTY of times that it is 100% ok to run 7.4v. They run same high voltage board inside. So 7955 over 7954 all day, titanium gears and there same price.

I guess the dog ate my homework.

We will have to agree to disagree. I too have talked to Hitec, after sending in a servo with a fried board and motor. I chose not to get the 7955 replacement, but a 7954 instead.

Good thing we have options to choose from.