Geared hydraulic flow dividers contain moving, rotating parts – keeping the speed within a good working range is important for optimum performance
The rotation speed depends on the capacity per revolution of the specific design, and the oil flow rate for each element in that design. While a hydraulic flow divider will generally work between 750 rpm to 3000 rpm, its optimum speed range is between 1000 rpm and 2000 rpm.
Flow divider elements each handle one division of the flow with the combination of elements being selected to achieve the desired oil flow within the speed range above.
This guide on selecting a hydraulic flow divider has been produced in conjunction with Slack & Parr. Their flow dividers are available with elements in several different sizes and in two distinct and discrete ranges:
- Slack & Parr FDR range: 2 | 4 | 6 | 9 cc/rev capacity
- FDL range: 12 | 18 | 24 | 30 cc/rev capacity
Flow dividers can be used in a number of different ways
Despite being constructed from the same standard components, Slack & Parr flow dividers can be used as an:
- Equal flow divider: to divide a supply flow into a number of equal separate streams (2 to 6 divisions is usual, but more can be achieved for special applications);
- Unequal flow divider to divide a supply flow into a number of unequal streams;
- A pressure intensifier can provide a higher pressure for part of a supply flow.
Each application needs a slightly different approach to be taken when calculating the sizing selection. For new systems design, the supply can also be defined after the flow divider. However, if a flow divider is to be retro-fitted into a system the existing supply must be taken into account.
Selecting a hydraulic flow divider for equal flow applications
In cases where the supply flow rate is known, the steps to be followed are:
- Divide the supply flow by the number of divisions required;
- Convert the flow units to cc/minute;
- Divide this cc/minute by the optimum speed of 1500 rpm;
- Select the nearest element size available;
- Check exact operating speed for this size
As an example, using a 2-way flow divider, to split the oil supply 50:50:
- For a supply flow of 1000 litres/hour, each half will be 500 litres/hour;
- 500 litres/hr (divide by 60 and multiply by 1000) is 8333 cc/minute;
- Dividing this 8333 cc/minute by the optimum mid-range speed of 1500 rpm (8333 / 1500) translates to 5.55 cc/rev;
- The nearest element size available in this case is 6 cc/rev;
- We can check the exact operating speed by dividing the split oil flow (here, 8333 cc/min) by 6 cc/rev: 8333 / 6 = 1388 rpm which is within the optimum speed range.
In cases where the individual delivery flow rate is known:
- Convert the flow units to cc/minute;
- Divide this cc/minute by the optimum speed of 1500 rpm;
- Select the nearest element size available;
- Check the exact operating speed for this size
Selecting a flow divider for unequal flow applications
The principle of selection is similar to that for equal flow dividers:
- For each unequal flow rate pick an element size that gives the nearest to 1500 rpm;
- Bear in mind that each element will rotate at the same speed as the others, and also that FDR flow divider elements cannot be mixed with FDL elements;
- Consider what combinations of element sizes will give the ratios of flow needed – three equal elements can give a 2 to 1 ratio for instance;
- After selecting the element numbers and sizes, re-check the speed is within the optimum (or if necessary, the working) speed range.
Alternatively, our technical team will be able to help you select the right configuration for your required flow ratio.
Let’s work through an example, using a 2-way flow divider to split the oil supply 60:40:
- For a supply flow of 1000 litres/hour, one side will be 600 litres/hour and one side will be 400 litres/hour
- Firstly, 600 litres/hour (divide by 60 and multiply by 1000) equates to 10000 cc/minute
- Divide this cc/min (10,000) by the mid-range (optimum) speed of 1500 rpm (10000 / 1500) = 6.66 cc/rev.
- Then 400 litres/hour (divide by 60 and multiply by 1000) equates to 6666 cc/minute
- Divide this cc/min (6666) by the mid-range (optimum) speed of 1500 rpm (6666 / 1500) = 4.44 cc/rev.
In this case, the nearest available element sizes are 6 cc/rev and 4cc/rev.
- Bearing in mind that each element will rotate at the same speed as the others, check exact operating speed: Divide the split oil flow (10000 cc/min) by 6 cc/rev: 10000 / 6 = 1666 rpm.
- The 4cc will automatically be rotating at the same speed.
- This is within the optimum speed range.
Selecting for pressure intensification
If all but one of the outlet flows is piped to tank (no outlet pressure), the energy gained (from supply pressure) through the flow divider can be used to intensify the pressure of the remaining flow.
The basic equation for two elements is:
P1 = (E1 x Ps + E2 x Ps – E2 x P2)/E1
Including the effect of PR, used to rotate the flow divider:
P1 = ((E1 x Ps + E2 x Ps – E2 x P2)/E1) – PR
Where: PR = pressure to rotate the flow divider (assumed 17 Bar); P1 = outlet pressure at high pressure element; P2 = outlet pressure at low pressure element; Ps = supply/inlet pressure; E1 = element capacity at high pressure element; E2 = element capacity at low pressure element.
For example, if an equal 9cc flow divider is used, with a supply pressure of 100 bar, and P2 is returned to tank (0 bar):
P1 = ((9 x 100 + 9 x 100 – 9 x 0)/9) – 17
P1 = 200 – 17 = 183 bar
Achieving more than enough pressure is also possible through using a reduced flow rate. By selecting the right ratio of element sizes the flow rate can be optimised while obtaining adequate pressure. Different capacities of elements can be used to obtain more or less intensification.
Feel free to contact us for further technical guidance in selecting and configuring the right hydraulic flow divider for your application – we’ll be delighted to help!
