VACUUM PUMP FAILURE! Are you neglecting yours?? - Page 8 - PeachParts Mercedes-Benz Forum

funola · #**106** 06-24-2012, 09:39 PM

My spare vac pump is a piston type, with one pipe, as stated in my 1st paragraph.

Quote:

Originally Posted by Stevo

Your diaphragm pump has only one pipe coming off it? Huh, All the diaphragm pumps that I have seen have one to the air cleaner and the other to the brake booster.

Quote: Originally Posted by funola
I have an old plastic vacuum hardline to get the male fitting from (it has 2 of them, one on each end). What I need is the female fitting such as the one on the brake booster or on the vacuum pump. Where did you get the female fitting from? Did you have to hacksaw it off? I am not near my spare piston vacuum pump or brake booster to look.

I see you have a diaphram pump with the 2 pipes. My spare vac pump has one pipe and only one fitting, which I do not want to destroy (if hacksawing is necessary).

Beagle · #**107** 06-25-2012, 06:55 AM

Quote:

Originally Posted by kerry

So the argument is that the piston 'floats' after it reaches a certain pressure, its stroke being substantially reduced by the effect of the vacuum on the rebound pressure of the spring?

Since I started this thread I guess I’d better stick around to see it out!

Not sure what you are getting at when you say “floats” ?? If the spring strength was 90lb and the piston (vac) pull was 45lb and the engine was turned off at that point the roller would be stationary at half stroke (in equilibrium). At half stroke the spring can only pull 45lb – it can’t pull the piston back any further as that would increase the piston’s vacuum and pull, it’s not floating it is locked there and going nowhere. Likewise when the pistons vacuum pull reaches 90lb and the spring fully compressed pulls 90lb the piston is locked fully forward (Parked).

Stevo · #**108** 06-25-2012, 09:53 AM

Quote:

Originally Posted by funola

I see you have a diaphram pump with the 2 pipes. My spare vac pump has one pipe and only one fitting, which I do not want to destroy (if hacksawing is necessary).

You need to make a trip to the wrecking yard with your 19mm wrench.

Stretch · #**109** 06-25-2012, 10:04 AM

I've been doing some rough calculations to get an idea of the effect a vacuum behind a piston might have. Can someone check my sums for me?

Calculation of the area (A) of a piston with a 70mm diameter (= 0.07m) is

0.07 m X pi

= 0.2198 m^2

(And pi = 3.14)

Pressure (P) of 1 bar = 100,000 pa = 750 mmHg = 14.5 psi = 29.5 "Hg

Putting the (metric versions of these) numbers into the relationship Force (F) = Pressure (P) X Area (A)

Force (F) = 100,000 Pa X 0.2198 m^2

Force (F) = 21.98 N = 2.24 kgf = 4.94 lbf

The relationship / equation above is assuming a frictionless perfectly sealed piston where the pressure (P) is the pressure difference on each side of the piston.

Please note this is a scalar quantity - no "plus" or "minus" sign has been used to indicate the direction of the force.

I plan to do some more complicated calculations - but I thought I'd check to first to see if I can get the basics right!

Brian Carlton · #**110** 06-25-2012, 10:12 AM

Quote:

Originally Posted by Army

I've been doing some rough calculations to get an idea of the effect a vacuum behind a piston might have. Can someone check my sums for me?

Calculation of the area (A) of a piston with a 70mm diameter (= 0.07m) is

0.07 m X pi

= 0.2198 m^2

(And pi = 3.14)

Pressure (P) of 1 bar = 100,000 pa = 750 mmHg = 14.5 psi = 29.5 "Hg

Putting the (metric versions of these) numbers into the relationship Force (F) = Pressure (P) X Area (A)

Force (F) = 100,000 Pa X 0.2198 m^2

Force (F) = 21.98 N = 2.24 kgf = 4.94 lbf

The relationship / equation above is assuming a frictionless perfectly sealed piston where the pressure (P) is the pressure difference on each side of the piston.

Please note this is a scalar quantity - no "plus" or "minus" sign has been used to indicate the direction of the force.

I plan to do some more complicated calculations - but I thought I'd check to first to see if I can get the basics right!

The piston is 70 mm or 2.75".

Resulting area is 5.96 in^2.

Pressure is 14.7 lb/in.^2

Force is 87.6 lb.

Beagle · #**111** 06-25-2012, 10:48 AM

0.07 m X pi

= 0.2198 m^2 No that’s the circumference!

(And pi = 3.14)

Pressure (P) of 1 bar = 100,000 pa = 750 mmHg = 14.5 psi = 29.5 "Hg

Better to keep everything in inches and psi Army. 70mmØ = 6in^2 within a cats whisker and that makes all the calculations mental arithmetic. Let’s keep it simple. It’s the dynamics that are disputed not the accuracy of the figures. I’m just using P = 15psi for simplicity.

qwerty · #**112** 06-25-2012, 11:22 AM

Quote:

Originally Posted by Brian Carlton

Pressure is 14.7 lb/in.^2

Force is 87.6 lb.

Not at 25" Hg, it isn't.

Stretch · #**113** 06-25-2012, 01:25 PM

Sorry about the melt down folks! I've just brought number one son back from foot ball training where I too realised I'd calculated the circumference not the area of a circle.

Doh.

Oh well - here's my metric calculations for the second time

Calculation of the area (A) of a piston with a 70mm diameter (= 0.07m) is

(0.07 m / 2)^2 X pi

= 0.0038465 m^2 = 5.962 square inches

(And pi = 3.14)

Pressure (P) of 1 bar = 100,000 pa = 750 mmHg = 14.5 psi = 29.5 "Hg

Putting the (metric versions of these) numbers into the relationship Force (F) = Pressure (P) X Area (A)

Force (F) = 100,000 Pa X 0.0038465 m^2

Force (F) = 384.65 N = 39.22 kgf = 86.47 lbf

The relationship / equation above is assuming a frictionless perfectly sealed piston where the pressure (P) is the pressure difference on each side of the piston.

Please note this is a scalar quantity - no "plus" or "minus" sign has been used to indicate the direction of the force.

Calculation for 25"hg = 0.8466 bar = 84,660 Pa

Comes out at 325.6 N = 33.2 kgf = 73.19 lbf

Stretch · #**114** 06-25-2012, 01:34 PM

Quote:

Originally Posted by Beagle

0.07 m X pi

= 0.2198 m^2 No that’s the circumference!

(And pi = 3.14)

Pressure (P) of 1 bar = 100,000 pa = 750 mmHg = 14.5 psi = 29.5 "Hg

Better to keep everything in inches and psi Army. 70mmØ = 6in^2 within a cats whisker and that makes all the calculations mental arithmetic. Let’s keep it simple. It’s the dynamics that are disputed not the accuracy of the figures. I’m just using P = 15psi for simplicity.

Thanks Beagle - I've never really used inches in anger - I'm of the metric age here. I'll present my answers in inches as well though.

I'll also try and figure out something for the dynamics. Though judging by my poor attempt at calculating the area of a circle I guess we'll be in for a few laughs! Boy am I rusty...

Stretch · #**115** 06-25-2012, 01:40 PM

...can anyone explain to me why it is harder to create a vacuum at higher altitudes?

If you go high enough you end up in space! The ambient pressure gets lower and lower the higher you go - how come it is harder to follow the trend?

Is it a pump efficiency thing? Not being able to move thin air masses?

qwerty · #**116** 06-25-2012, 02:40 PM

Quote:

Originally Posted by Army

...can anyone explain to me why it is harder to create a vacuum at higher altitudes?

It isn't harder to create a vacuum at higher altitudes. But, as altitude or elevation increases, the pressure differential decreases as atmospheric pressure decreases.

Stretch · #**117** 06-26-2012, 02:58 AM

Quote:

Originally Posted by qwerty

It isn't harder to create a vacuum at higher altitudes. But, as altitude or elevation increases, the pressure differential decreases as atmospheric pressure decreases.

I understand that - that's what I'm saying. The ambient air pressure is closer to vacuum the higher you go. So how come people have said in this thread that it is harder to make a vacuum at higher altitudes? How come that is also reported elsewhere on the internet?

(Should I provide links? And point the finger?!)

Stretch · #**118** 06-26-2012, 05:18 AM

Well whilst most of you have been sleeping I've hunting about for my vacuum pump! I can't find the sodding thing anywhere. My car is in bits so it is in a box somewhere - supposedly somewhere where it is easy to find because I wanted to rebuild it...

...anyway...

...if anyone can help (funola?) with the following data for the piston vacuum pump I'd be much obliged

1a) The thickness of the big return spring
1b) The number of coils of the big return spring
1c) The diameter of the coils (outer, inner, or mean diameter - it doesn't matter which)
1d) The distance between each coil when not compressed might also help a bit

2a) The thickness of the little return spring (that sits inside the big one)
2b) The number of coils of the little return spring
2c) The diameter of the coils (outer, inner, or mean diameter - it doesn't matter which)
2d) The distance between each coil when not compressed might also help a bit

3) The weight of everything that "floats" on the springs - so that's the piston, the connecting rod, the bearings and rolling wheel that runs on the cam on the timing device.

I realise this is a big ask and that it is only applicable to people who happen to be taking a piston vacuum pump to bits...

...if this data isn't forthcoming then I'll just have to wait until I find my own pump and pull that one to bits!

Stretch · #**119** 06-26-2012, 08:20 AM

Quote:

Originally Posted by Beagle

Army. Perhaps you would care to start a new thread for this??

Its totally off topic and irrelevant to this one. The math we require here would not challenge my 12 year old grandson.

Fair enough - will do.

(What I've got planned isn't off topic but I guess if you want to go and enjoy that Audi!)

qwerty · #**120** 06-26-2012, 03:38 PM

Quote:

Originally Posted by Army

I understand that - that's what I'm saying. The ambient air pressure is closer to vacuum the higher you go. So how come people have said in this thread that it is harder to make a vacuum at higher altitudes? How come that is also reported elsewhere on the internet?

(Should I provide links? And point the finger?!)

It all depends on whether you are dealing in absolute terms or relative terms. Your vacuum pump might be able to easily pull a perfect vacuum in space. Unfortunately, your 100% vacuum will create a pressure differential of zero.

So, it's absolutely easier to create a vacuum as altitude increases, but the relative differential is diminished. You cannot pull a one bar differential at the 500 millibar level. And you cannot pull 29" Hg in Denver. (Note that it isn't "harder" to do; it is impossible.)