Massey Ferguson 390 tandem pump repair

The new hydraulic pump purchased for Massey Ferguson 390 and the old pump, that sucked air pass the axle sealants. These pumps are so called external gear wheel pumps. The drive gear outside the pump had to be dismantled in order to change the axle sealants. It was difficult, the gear was apparently glued to the conical shaft. I warmed the gear with hot air gun and tightened the puller beyond reasonable limits. Then hit the key in axle slot with a punch and the gear finally let go. I could have at first taken the pump to pieces and pressed the gear away against the end plate, maybe easier this way. The shaft is conical and about 17,5 mm of root diameter.

The low pressure pump section separated. The pump gears are connected with splined sleeve taken off it's right place.

The high pressure pump section and the pumping gears inside it.

The pump housing includes also swimming bearing elements. The gears have eaten surface of the housing a bit.

In this pump the sealing O-rings are in the end plates, in some hydraulic pumps they are in the bearing elements. Pressurized oil can get behind bearing elements and press them suitably against gears, which reduces spills back to inlet. On the other hand lubricating oil between axle and bearing element can get back to null pressure side. Pressure side is on the right.

I destroyed threads in the drive shaft when removing the helical gear wheel, but managed to file them enough to make the nut draw. The upper gear in the picture is called drive pump gear and lower idler pump gear. Then there is this helical gear, that makes both pumps to go around, confusing.

Axle sealants behind the drive gear leaked air into the pump. Pumped oil was very foamy and nothing worked.

The seal kit I bought includes two axle seals. The heart shaped O-rings are enclosed with support rings.

My tandem pump had two kinds of heart shaped sealants, but sealants in the kit were all similar. I assembled the pump together partly with used sealants and will buy a new pump, if this don't work. The most important issue is, that this tandem pump circuit works also in the Massey Ferguson 565.

MF565 hydraulics
MF390 hydraulics
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