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MJX T-634 Lower rotor doesn't rotate...


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kwikphix328
Hover Master
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Joined: 30 Dec 2011
Posts: 8
Location: Scotland, Dundee area.

PostPosted: Sun Jan 01, 2012 11:28 pm
PostPost subject: MJX T-634 Lower rotor doesn't rotate...
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Hello Folks,

First post here, hope you can help.

My Girlfriend bought me one of these Helicoptors for Christmas. I did manage to get it off the ground in fairly non controlled flight Smile.
( Tried Running before i could walk ! )

Think i have had one crash too many though. I dropped the remote and had to re-attach the throttle potentiometer which had pinged off inside !

With this fixed I managed to get back in the air again however even though i was in the middle of a huge field i managed to crash the Heli into a concrete yard from fairly high altitude.

The symptoms i have now are as follows:

i) Heli powers up, upper Rotor rotates at what looks like full speed, lower rotor is obviously much slower and some times doesn't rotate at all. ( i think itmight even be rotating due to downdraught from above).

ii) If i hold the Heli by the skids and rotate the upper rotor the gear/motor shaft inside rotates, i can feel it 'cogging'.

iii) If i rotate the lower rotor the gears/motor shaft sometimes rotate and then sometimes just 'free wheel' with no cogging felt.

I am assuming that the transmission drive line between the lower rotor and its electric motor has eithe rbecome loose or broken.

Any advice for a fix, or how to isolate the specific fault so i can get some new parts if required please.

I work on commercial passenger aircraft for living, however these model flying machines have me beat at the moment Smile

Regards
Tim.
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pjdog
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Joined: 12 Jan 2010
Posts: 2073
Location: Hudson, Florida, USA

PostPosted: Mon Jan 02, 2012 12:42 am
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What you did is what we have all done at one time or another.

Sense you work on full size stuff this should be a piece of cake for you to fix.

First be sure that the motors ( I think your helicopter will have two) for your main rorot blades are making contact with the main rotor gears. Each motor will have a pinion gear that should match up to a large main gear. If that is good then be sure that you have not dis-loged a grub screw that sets the main rotor gears to the main rotor shafts. Some times in a good crash this can happen. If this is the case you will need to get a allen set that will match the grub screw and find the flat spot where they should be set.

Lastly I hope you have not damanged the 4-in-1 controller. This little thing controls the power to each motor. If when you crash the first thing to do even before you hit the ground is drop all the power to off. If power is still on when you crash the 4-in-1 will get over loaded with power and no where to send the power because of motor blockage so an electronic part will pop. Now your in trouble. The 4-in-1 will need to be replaced. In most cases of lower priced rc helicopters it's almost a cost effectivity to replace the helicopter as to purchase another 4-in-1. With higher priced helicopters the replacement is worth it. That's a decision you'll need to make.

Please let us know how you do so we can keep up with the result. Lots of times when one gets back in the air we never know what happened.

Good luck with you efforts and happy new year.

And welcome to the forum.

Jack
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kwikphix328
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Joined: 30 Dec 2011
Posts: 8
Location: Scotland, Dundee area.

PostPosted: Mon Jan 02, 2012 4:57 am
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Thanks for your reply Jack.

Bit more info:

if i hold the Heli by the skids and hang it upside down i can see both of the electric motors running and driving both of the main gear drives. I can hold the lower rotor still with my finger though and it has no effect on the motor so i know that the connection between the main gear and the rotor drive shaft has become loose.

Looks time consuming and difficult to get into the guts of these little machines though.
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tombo242
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Joined: 04 Nov 2008
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Location: Santo Estêvão, East Algarve, Portugal. Now 82, but still feels 22.

PostPosted: Mon Jan 02, 2012 5:24 am
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Hi Kwikphix,

Looks like yu've found the problem there. A possible fix coult be a spot of CA or loctite on the gear/motor shaft. whatever you use don't let it get onto any gear teeth.

Tom.
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pjdog
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Joined: 12 Jan 2010
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Location: Hudson, Florida, USA

PostPosted: Mon Jan 02, 2012 8:22 am
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kwikphix328


I think you need to look for a grub screw that holds the main gear to the main shaft. Sounds like it's broken or come lose in the crash.

Anyway like Tom said that's the problem. Motor is driving the main gear but the main gear is not getting to the main shaft. Ther must be something that hooks the two up. Find it and fix it.

jack
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kwikphix328
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Joined: 30 Dec 2011
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Location: Scotland, Dundee area.

PostPosted: Wed Jan 18, 2012 7:08 pm
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Well it has taken me a while as i have been busy working, however my hei has been stripped down, repaired and rebuilt.

the problem was caused by the two small screws that attach the lower rotor gear to the drive shaft had sheared. Just a case of finding some suitable replacement bolts ( M2 machine screws) cutting them down to size and then refitting everything.

Found a great RC Model helicoptor shop just down the road from my house in Carnoustie. ( about 1 mile away how lucky is that !) The guy who runs it is a real enthusiast and was most helpful. I am sure i will be paying him more visits in the future as he had some really nice 'hobby grade' heli's in store.

He even let me have a go of one of his heli's and i managed to get it to hover.... for about 2 seconds Smile
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tombo242
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Location: Santo Estêvão, East Algarve, Portugal. Now 82, but still feels 22.

PostPosted: Wed Jan 18, 2012 7:58 pm
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That's great news Tim.

Tom.
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