FEATURE CLUTCHES, BRAKES & COUPLINGS Torque about coupling specification


Last April, Huco Dynatork wrote the first ‘How to..’ guide for Design Solutions, which explained how to select the best coupling for true angular misalignment, zero misalignment and axial compliance. In this latest installment, the company explains how to select the best coupling for torque capacity and torsional stiffness


W


hen it comes to selecting a coupling for an application, many points


need to be considered. In this instance, however, we will concentrate simply on selecting the best type of coupling for torque capacity and torsional stiffness. Torque is the angular force needed to


overcome the resistance of a load. Rotating loads have both a friction and an inertial component and are classified according to whichever dominates. As an example, the resistance encountered by a pump delivering fluid is a frictional load as the inertial part is secondary, assuming that the pump runs continuously at a steady speed. The total application torque comprises the frictional plus inertial elements.


If


the pump runs at a constant speed it produces a uniform load and the required power would be given in kW or HP. The kW rating is related to torque by the following formula: torque Nm = kW x 9550 divided by revolutions per minute. Conversely, a ball-mounted slide table,


typified by short cycles of rapid acceleration and deceleration in both directions of rotation, will have inertial


Mode: Motor accelerates load. Torque ‘seen’ by coupling = load inertia + frictional resistance of load


Mode: Motor decelerates load. Torque ‘seen’ by coupling (in opposite direction) = load inertia - friction resistance of load. The coupling ‘sees’ this as a torque reversal although the direction of rotation is unchanged


TORSIONAL STIFFNESS Although torsional stiffness may be expressed in different units, the most common and easiest to work with is Nm/rad. Often described as torque per unit deflection, torsional stiffness is significant in positional system


loads as the predominant factor. These will determine the reversing torque factor of the coupling. To be more precise, the maximum


torque experienced by the coupling may be dictated by whether braking is applied by the load or the motor. In the diagrams below the arrows indicate the direction of the angular forces due to acceleration, deceleration or braking. Once the maximum torque in the


system is known, the selection of the correct coupling can be made by relating it to the peak torque rating. The coupling should be selected using the following formula: peak torque ≥ application torque x service factor.


and describes a coupling’s resistance to torsional deflection. The inverse of torsional


stiffness, torsional deflection is defined by deflection per unit torque. This also has many denominations but is best expressed in degrees/Nm. When used in a closed


loop or velocity control system, a coupling’s torsional stiffness becomes more critical and forms a contributory factor in calculating the upper limit of dynamic performance and stability. The stiffness of a coupling should therefore be such that its torsional resonance frequency exceeds 300 – 600 Hz, depending on dynamics. While stiffness is at its most critical


when load inertia is dominant, it becomes less so when the dominance swings in the motor’s favour. Taking into account factors including


those described above will help to ensure the coupling meets the demands of the application it will be used in.


Huco Dynatork www.huco.com Enter 217


ENSURING TRUCK SAFETY ON SLIPPERY ROADS


Faced with slippery winter roads and downhill gradients, trucks with trailers face the risk of instability and could even jack-knife. To help improve safety, Volvo Trucks has developed the Stretch Brake system which, the company claims, automatically retards the trailer and straightens up the rig on slippery downhill stretches. Stretch Brake is a complement to the rig’s


electronic stability program (ESP). While ESP is at its most effective at higher speeds, Stretch Brake is only operational at speeds below 40km/h. Both systems contribute to better stability and easier steering. “One might call Stretch Brake a kind of


low-speed ESP. As the rig approaches a downhill slope, the driver manually activates the system.


When the driver then releases the accelerator, the brakes on the trailer are automatically applied in a pulsated mode all the way down the hill until the gradient levels out and speed can once again be increased,” explains Mats Sabelström, brake specialist for the Volvo Trucks brand. According to the Volvo Trucks Accident


Research Team, which specialises in studying traffic safety, about 60 or so of the truck accidents that occurred in Sweden alone last year could have been avoided with Stretch Brake. The solution increases safety on downhill


gradients, especially on slippery roads and in curves; applies the drawbar brakes in a pulsating mode; and straightens the rig, making steering easier and reducing the risk of jack-knifing.


It was introduced in 2012 on Volvo FH


trucks pulling drawbar trailers and in 2013 on Volvo FM trucks pulling drawbar trailers. In 2014 it will also become available for Volvo FH and FM semi-trailer rigs.


Volvo Trucks www.volvotrucks.com Enter 218


The formulae for torsional stiffness and resonant frequency


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APRIL 2014 | DESIGN SOLUTIONS


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