Hi everyone!
Today’s topic might seem to some people even spookier than Halloween ghosts, and it’s all about choosing the right PTO. I’ll explain how PTO specialists use the information they gather from you to calculate the specifications and select the correct PTO.
But before we delve into this topic, here’s an industry joke.
Why are torque and gear ratio like husband and wife? Because torque is what makes things turn, and gear ratio is what tells you how fast to turn them;)
The first piece of information we need to start selecting a PTO is the chassis make and gearbox type. The gearbox label is essential; without it, we’re as lost as children in a fog. The salesperson uses the information from the plate to check the PTO manufacturer’s catalogue for the available PTO options for that specific gearbox type and, using the other information you provide, tries to select the best option.
In their catalogues, manufacturers provide information such as:
– PTO mounting position (e.g. top/bottom, rear, side, front)
– control type (e.g. pneumatic, mechanical, electrical)
– direction of rotation (remember! PTO – CLOCKWISE, PUMP – COUNTER-CLOCKWISE)
– type of outlet (e.g. flange, 4H)
– torque and power
– gear ratios for low and high ranges
We’ll take a closer look at torque and power, as it’s very important to calculate the torque and power required, for example, for a pump, in order to avoid overloading the PTO.
The torque is calculated taking into account the pump’s capacity and the system’s operating pressure, as well as an additional factor specified by PTO manufacturers.
Torque is calculated using the following formula (which takes into account the pump’s average mechanical efficiency):
M = q × p / 60 (or 56, depending on the manufacturer)
M – TORQUE [Nm]
q – PUMP DISPLACEMENT – [cm³/rev]
p – SYSTEM OPERATING PRESSURE [bar]
For example, what is the required torque for a system operating at a pressure of 190 bar and with a pump delivering 90 cm³/rev?
M = 90 × 190 ÷ 60 = 285 Nm
And what will the output power of such a PTO be?
It is calculated taking into account the pump's capacity, the PTO ratio, engine speed and operating pressure.
P = q × n × p / 60 × 0.79 × 1000
P – POWER (kW)
q – PUMP DISPLACEMENT – [cm³/rev]
z – PTO RATIO
n – ENGINE SPEED (rpm)
p – SYSTEM OPERATING PRESSURE (bar)
0.79 – PUMP EFFICIENCY COEFFICIENT
For example, in the same system with a pressure of 190 bar, a 90 cm³/rev pump driven by a PTO at 900 rpm, the output power at the drive shaft will be
P = 90 × 900 × 190 ÷ 60 × 0.79 × 1000 = 32.46 kW
If the calculated values of the coefficients exceed the maximum permitted values for the PTO, a different system configuration must be used.
It is also very important to take into account the operating time and output speed when making these calculations.
PTO catalogues specify the usable torque, typically based on operation at up to 1,500 rpm for up to 15 minutes.
As the engine speed and operating time increase, the available PTO torque decreases; for example, after 15 minutes of operation, it decreases by 15%
up to 1500 rpm = 100% up to 15 minutes = 100%
from 1500 to 1750 rpm = 85% from 15 to 30 minutes = 85%
from 1750 to 2000 rpm = 70% from 30 to 60 min = 60%
from 2000 to 2250 rpm = 50% for over 60 minutes = 50%
For example, a power take-off unit with a rated output torque of 350 Nm.
at a speed of approx. 1750 rpm
and PTO load time = approx. 30 minutes.
The torque that this power take-off unit can deliver under these operating conditions will be
350 Nm × 851 TP3T × 851 TP3T = 253 Nm.
As I mentioned earlier, an important parameter of a PTO is its gear ratio. Using this parameter, we can calculate the system’s flow rate. The PTO output shaft speeds given in the catalogues refer to an engine speed of 1000 rpm.
For example, what will the oil flow rate be for a system of this type with a pump rated at 90 cm³/rev and a PTO with a gear ratio of 1300 rpm (1:1.3)? We calculate this by taking into account the pump capacity, the PTO ratio and the engine speed at which we wish to operate (usually 800 rpm)
Q=[q*n1*(n2/1000)]/1000
Q – system flow rate [l/min]
q – pump capacity (cm³/rev).
n1 – rotational speed of the PTO output shaft (value specified in the catalogue)
n2 – the engine speed at which we wish to operate
Q = [90 × 1300 × (800/1000)] / 1000 = 93.6 l/min
For the example given, the flow rate will be 93.6 l/min.
Right, right, I get that not everyone’s keen on doing the maths and calculating PTO parameters themselves. But don’t worry, there’s always a way round it!
If the clock on the wall seems to be racing against you, and numbers feel more like a puzzle than a pleasure, I’m here to help! All you need to do is give me a call or send me a message. You’ll find all my contact details on our Trucks Concept website. I look forward to your calls and messages, ready to get the calculations rolling!
May maths always be on your side, and on mine – calculations, computations and selection. Everyone is welcome!
See you next Monday! Stay tuned.
Your Asia!
