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Thursday, February 21, 2019

Why correctly Matching Power Speed and Pump Flow is so important in Hydraulic Lift Systems

The Original title of this blog was 
Warning How Not to Lift a 4200 Kg Forklift Vertically 6 m in 8.5 seconds.

That's moving 4.2 tons at a velocity of 70 cm per second or 2 feet 3-3/4 inches per second straight up.

Just before Christmas 2018, we were called out to investigate and make emergency repairs to a hydraulic goods lift. The 47 KW (60 HP) 2800 RPM high-quality Italian made Elmo electric drive motor designed to operate fully submerged in oil had failed. 

The Motor 
The electric motor rewind shop the motor was sent to told us that it was a catastrophic failure and it was not repairable. They also commented that they had not seen such an unusual motor failure before. The Failure looked like a high amp weld or spark had jumped across the motor casing and into the rotor housing and in the process had burned out a piece of the rotor.  
Further investigations in the new year revealed that a new motor was not available in the country. (Australia). And that a replacement motor would cost Au $13,000 and would take 6 weeks to get here. 

The Pump
The hydraulic pump installed was a beautifully designed high flow Italian S M screw pump and was sized to flow up to 660 Liters of oil per minute at 2800 RPM at a maximum pressure of 40 Bar (590 PSI)

Beautiful Internals of the 660 lt/min hydraulic screw pump.
this pump is for sale contact us below. 

  • A German-made high-quality flow control valve manufactured by Blain Hydraulics.
  •  Click here to see the quality and versatility of this awesome valve 
  • A 2" shut of valve with a built-in emergency lowering valve 
  • A set of large bore hydraulic hoses 
  • A huge gear flow divider 
  • A pair of identical three (3) stage telescopic cylinders.
  • And an oversized 800-liter hydraulic tank. 

The Dramas and the Stresses
I will go into the details at a later date but for all you fluid power engineers and hydraulics specialists out there, our quick fix to get the customer's equipment going again caused us major grief, stress, and embarrassment because..... we assumed that the people who had installed the hydraulic system including the pump and drive motor (and had also inspected it for many years) knew what they were doing !! 
And as our client kept repeating SH*@$#@T the lift has worked without drama for many years!!    

The exposed well of Hydraulic Goods lift with Twin 3 stage Telescopic Cylinders
and Gear Flow Divider between them 

The Quick Fix?

As the original design HAD THE PUMP AND MOTOR FULLY SUBMERGED IN THE OIL  
Our quick fix was to rotate the pump through 90 degrees mount it to a new fabricated  support frame and drive it using a new readily available 45 KW air cooled motor using extended drive shaft and solid couplings. So to clarify the Pump is still submerged below tank oil level but the drive motor is dry and out of the oil and is mounted above the tank and is driving the pump with an extension shaft.  

It took us about 4 days to source new motor, frame materials and then fabricate the frame, mount motor and pump, machine up extension shaft and then organise an electrician.

Commissioning Dramas  All went well until commissioning and testing of the new system. The new motor was struggling to power the pump and was just managing to raise the lift frame empty it was also drawing massive amounts of current (the Maximum rated continuous current  of the new motor was 76 Amps and was drawing up to 170 Amps) this high amps was even there after switching from Star to Delta so much so that is was going out on overload. This was quite strange because the old Elmo Motor and the new motor where quite well matched in Power Current draw and RPM. 
The motor was close to overheating and the oil was getting very warm, so a lot of energy was going somewhere??? 

Trouble Shooting
So without thinking it through quite thoroughly enough we thought "something must be wrong with the flow control valve or there is something stuck in it restricting flow" So we bypassed the valve and ran the motor and pump on recirculate, all ran well and with only 30 amps drawn so just to make sure we ran the system for 30 minutes with minimal temperature increases. So I thought It must be valve.

The Valve 
We thought about bypassing the valve and raising the lift with the pump only but thought better not as there was no real way of controlling the flow and speed without a lot of additional hoses and valves and too much risk.

So out the valve came and it was dismantled to pieces,  Did we find anything stuck inside ? No not really, there was some wear and tear from age and contaminated oil ( the entire Blain EV100 valve is made from Aluminium) and a few tired and slack O rings (actually lots of tired and slack O rings) but nothing that indicated a blockage, perhaps we thought the tired O rings where preventing the valve from working correctly so we replaced all that we could reassembled and set up all the default settings and the same thing, High amps, heat and eventual over load.  

I called the electrician he checked and double checked everything and declared all was good something was overloading the motor, we checked the extention shaft and couplings and bearings in motor and pump all OK. 

The Right Way to check a Hydraulic system's design.  

1. Get a pressure gauge and measure the pressure required to raise the lift empty and with forklift loaded on, so I modified a large elbow fitting after the Blain Control valve and drilled and tapped a hole for a standard test port. And then measured the pressures. 
31 bar to raise lift empty, 57 bar to raise lift and forklift.  

The above is done in trouble shooting mode, if designing from scratch you would have to calculate frame weight, payload weights, cylinder bore / rod diameters, pressure requirements and speed requirements and also allow for efficiency losses which can be as high as 40%  to 50% in some instances.

2. Check Pump Specifications as supplied by the manufacturer.
The pump was a Settima SMIT GR70 660 Lt 
according to the specifications this pump at 2800 RPM could pump 660 litres per minute of ISO 68 grade oil at a maximum pressure of 40 Bar. so we had 2 problems. 

  • We did not have enough power for such a large flow pump, according to specifications you would need 55 KW at 45 Bar but we needed 57 Bar and we only had 45 KW
  • Also the pump was the wrong pressure rating as it was only rated for a maximum of 40 BAR and we needed 57 bar to raise the lift frame and forklift. Further research indicated that to achieve 57 Bar at 660 Lt/min we would need a least 78 KW.
  • For the above reasons we recommended to the customer to change out the pump with a smaller flow, higher pressure to more closely match motor power available. With costs and time by this stage were blowing out we eventually chose a GR45 180 Lt this pump at full 80 bar pressure would only require 30 KW

Below is the full report of our findings as reported to the customer after a few days of testing, head-scratching, number crunching and research. 
The Blain Hydraulics online Lift Calculator was a great help in getting the customer to understand the situation and to also reinforced our findings research and calculations. 

For privacy reasons, I have removed all mention of the customer and location. 

Summary of Findings, Damaged original Drive motor & poor performance of replacement motor at XXXXXX ... XXXXX.

  • Calculated weight of empty lift is approx. 2200 Kg
  • Add 2000 KG for Forklift total weight 4200 KG.
  • The pressure required to raise empty lift = 31 Bar (450 PSI)
  • The pressure required to raise lift with forklift = 57 Bar (850 PSI)
  • The installed pump is rated to 40 Bar (57 bar required to raise lift and forklift)
  • Originally installed Pump Drive motor was rated to 47kw at 2770 RPM and 89 AMPS. 
  • New motor rated at 45 KW at 2960 RPM. (%7 higher speed) at 76.3 amps.
  • Power required to raise empty lift 45 KW
  • Power required to raise lift with forklift only! Is 81 KW.
  • Option 1 replace 2 pole motor with 4 pole motor to reduce speed to 1450 RPM.  However, the pump is working at 40% over the maximum rated pressure. And may not last long term.
  • Option 2 Best long term fix, source smaller pump rated at the correct pressure and installed motor speed.

·       Please see full calculations and pictures below.


·       Figure 1. Name Plate From Original Elmo Damaged Drive Motor 47 KW (64 HP) at 2770 RPM Max running amps 89 A
Figure 2. Name Plate From Replacement Techtop Drive Motor 45 KW (60 HP) at 2800 RPM Max running amps 76A. 










 Figure 3. Installed pump in tank, Specification is as follows
S M Screw pump GR70 –SM-660 LT
this pump is rated to only 40 Bar, 57 Bar is required to raise lift frame and forklift only.  


Figure 4. The Installed high Flow Pump requires 81 KW at 2960 RPM to flow 660 LT / Min.
It Requires 41 KW at 1450 RPM to flow 330 LT/Min.  See calculations below. 



Figure 5. Full Calculation Based on Actual Measurements  & Calculated Data. Motor Power required is 81 KW. Actual installed power before failure was 47 KW. Hence the catastrophic failure of the submerged electric motor after years of short duration high amp operation. 





Conclusion, today we are replacing the 660 liters per minute 40 bar pump with a higher pressure (80 Bar 1100 PSI)  pump rated to flow 180 liters per minute at 2800 RPM.
We will leave the new 47 KW, 2800 RPM replacement motor in place. 

When complete this installation will be capable of raising the lift superstructure, forklift and a ton of payload (total of 5200 Kg ) to the next level (6 meters up) in a little over 30 seconds.

New high pressure 180 Lt/min pump on left with old low pressure 660 Lt/min Pump on right. Bernardo our always smiling hydraulics technician working on the changeover.  

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