The Riverhawk Hydraulic Crosshead Nut is a hydraulic nut designed for long-service life.  A hydraulic nut provides a quick, efficient method of producing and retaining high clamping forces (preload) in the crosshead joint.  This is accomplished by connecting a hose from a high-pressure pump kit to the port in the hydraulic nut piston.  The pressurized oil fills the annular area inside the hydraulic nut, generating a large, purely axial load in the piston rod.  This load stretches the piston rod which creates a gap between the cylinder and retainer.  A wrench is provided to tighten down the retaining collar against the cylinder.  Once the hydraulic pressure or pressurized oil is released, the load is held back by the hydraulic nut.

A common question we often receive is, where is a Crosshead Nut used?  The Riverhawk Hydraulic Crosshead Nut is used on reciprocating compressors for petroleum chemical and gas industries.  The crosshead nut gets its name from the location it is used within the reciprocating compressor.  This is located on the crosshead slide.  The hydraulic crosshead nut connects the crosshead slide, which connects to the piston rod.  This is a very challenging joint to tighten due to space limitations.

• Significantly reduce installation and removal times.
• Increased preload accuracy and repeatability- allows OEM’s to potentially uprate their machine providing better efficiency.
• Reduced likelihood of piston rod failures.
• Uniform, pure tension loading eliminates bending and twisting moments that can be induced into the piston rod by traditional torque methods.
• No additional bending loads in piston rod due to off axis nut loading.
• Reduces the likelihood of fretting between the bottom of the nut and crosshead face.
• Eliminates high pressure sliding surface on threads of piston rods, extending rod life.
• Improved operator safety- eliminates slugging wrench so no more hammers or cranes.
• Hydraulic nut is optimized for alternating loads with high safety factor against fatigue failures.
• Counter-weighted crosshead nuts are custom designed to match the weight of existing nuts and/or counterweights (used for balance).

What is a Coupling Hub?

It is necessary over the course of large-scale turbomachinery’s life to perform maintenance to ensure optimal performance. One such service may be the repair or replacement of a coupling hub that is joined between two pieces of machinery.

The coupling hub serves as a way for a motor to provide torque to a compressor or similar equipment. This makes it essential to the overall performance of the machinery.

Determining the appropriate method to safely disassemble the coupling hub will reduce the risk of damaging the hub and its mating equipment, as well as reducing the amount of time spent servicing the machinery.

Differences in Hub Design

Coupling hubs vary in design depending on the manufacturer, offering a number of ways in which it can be installed or removed.

There are two main designs that an end user can encounter: a straight bore hub and a tapered bore hub.

A tapered bore hub consists of a small internal diameter on one end that gradually increases across the length of the coupling hub. A tapered hub is installed to a mating tapered shaft end such that a pre-determined interference fit between the two surfaces is attained. This style of hub is easily installed and removed through the use of a hydraulic tool such as a Riverhawk hydraulic pusher.

A straight bore hub possesses a consistent internal diameter across the hub length and is typically shrink fit onto a shaft end to generate contact pressure. Unlike the tapered bore coupling hub, a straight bore coupling hub can be removed by a number of different methods depending on the discretion of the end user and the equipment that is available.

Methods of Removal

1. Hydraulic Removal

The most common method of removing a straight bore coupling hub is through the use of a hydraulic removal tool such as the Riverhawk hydraulic puller.

The hydraulic removal tool typically consists of two main components: a cylinder and a piston.

The removal tool is attached to the hub face via tapped holes and utilizes hydraulic fluid to move the piston axially, which in turn will shift the hub axially in the opposite direction (off of the shaft end).

The primary benefits to this method of removal are speed and accuracy; installing and operating the hydraulic tool is simple and the user can accurately measure the amount of fluid injected into the tool and track the movement of the hub as it advances off of the shaft end.

This leads to less time spent on the job, which in turn translates to added cost savings for the end user.

There is a higher purchase cost associated with a hydraulic removal tool as a hydraulic pump and hose kit is required for operation, however this is offset by the decrease in time spent on the job and the repeatability of using said equipment.

2. Mechanical Removal

Another conventional method of hub removal is through the use of a mechanical removal tool.

The mechanical removal tool typically consists of a claw or similar device, and a threaded piston.

The tool itself behaves similar to the hydraulic tool, in that it is secured to the hub face or flange and utilizes a piston to engage with the shaft end and shift the hub axially.

The primary difference is that the tool requires the end user to turn the piston via a wrench or similar mechanical input.

While the purchase cost of the mechanical tool is lower than that of a hydraulic removal tool, the added time and labor costs may prove this to be the less cost-efficient method.

Additionally, the mechanical device is limited by the amount of torque that the user can input when engaging the piston; the tool may not be able to generate sufficient force to fully remove the coupling hub.

    3. Destructive Removal

The third method of hub removal is through the use of a plasma torch or similar device to remove material from the hub barrel with the intent to split the hub and relieve the contact pressure.

Caution should be taken when using this method as it is easy to remove too much material from the hub and begin to damage the shaft end.

Furthermore, the hub will release a substantial amount of force once separated, potentially causing injury to those in proximity.

As this method of removal will render the hub unusable, it should only be used when other removal options are not valid or if the hub is in severe disrepair and requires replacement.

Other Factors to Consider

• Proximity to Equipment – There may be limited space to install and operate a removal tool depending on the location of the coupling hub relative to its surrounding equipment.
• Environment – The coupling hub may be present in a hazardous environment that limits which method of removal to use.
• Dilation Assist – The end user may have access to different methods to dilate the coupling hub and reduce the contact pressure, be it hydraulic fluid or heat.

By reviewing the information above, you can significantly reduce the time spent identifying and selecting the appropriate method of removal for your coupling hub.

You may find other concerns that were not listed in this article. If that is the case, Riverhawk staffs a number of qualified engineers whom can address these concerns and help guide you in selecting which method is right for the job.

Let’s start this discussion by introducing a term used in hydraulic bolt tensioning called elastic recovery, or more commonly just called relaxation; which is the difference between force applied by a hydraulic bolt tensioner and the residual preload left in the bolted joint after the nut is turned down and hydraulic pressure is released. As the load is transferred from the tensioner to the bolt the following happens:

• Elastic embedment of previously non-loaded joint members
• Strain between the threads of the bolt and nut as they take the load
• Localized plastic deformation of machining crests. This can occur between thread surfaces as well as under the nut against the flange or any metal contact areas.

The amount of relaxation is directly correlated to the stiffness (geometry) of the joint. Stiffer joint elements will have less relaxation than those in which joint elements are allowed to flex. The opposite is true for the bolt, with long slender bolts having less effect on joint relaxation when compared with short bolts with high stiffness. In the most common bolted joint arraignments, the bolt effective length has the largest impact on the relaxation ratio and is used to estimate the amount of relaxation during the bolt tensioning assembly process.

So, what is the effective length of a bolted joint and how is it measured? Let’s start with clamp length or what is sometimes called grip length. This refers to the combined thickness of all members which are clamped together. This typically means, the length between the surfaces of the bolt head and nut. Note: any washers or spacer lengths would be included in the clamp length.

Effective length is the length of the bolt effected by the tension load and is used to calculate joint stiffness and bolt stretch. This is commonly equal to the clamp length plus ½ of the thread engagement length with the nut(s) or flange.

-In the Figure below, “C” represents clamp length and “L” represents the effective length.

As shown in the table, as the effective stud length to diameter ratio increases, the relaxation factor dramatically decreases until a ratio of 12 is reached. After such relaxation is not affected immensely and any further affect is neglected while estimating relaxation.

For example; a 2.00” stud with an effective length of 12.00” would have and L/D ratio of 6 (12.00/2.00=6). Following the chart above would have a relaxation factor of 1.175. Therefore, if a preload force of 83,000 lbf is needed, 83,000 lbf would be multiplied by 1.175 to find a load of 97,525 lbf which must be applied by the bolt tensioner to the stud in, order to achieve the in-service preload of 83,000-lbf.

±5% Accuracy can typically be achieved in bolts loaded with hydraulic bolt tension tools. So, the above load of 83,000 lbf could very well be between 78850-89640 lbf. If more precision is needed to control the load, stretch measurements can be used to accurately measure the load applied. A ±1-2% accuracy can be achieved when applying accurate stretch controls in conjunction with hydraulic bolt tensioning.  Once bolt stretch is verified during the initial installation, repeatability is very good for subsequent studs / assemblies using hydraulic pressure alone.

The same target load achieved by simple torquing typically could see a range of ±25-30% (Ref: An Introduction to the Design and Behavior of Bolted Joints, 3^rd Edition, John H. Bickford), So that load of 83,000 lbf could see a spread between 62,250-103,750 lbf (25%). Again, if more precision is needed to control said load, stretch measurements can be used to accurately measure the load applied, but unlike the tensioning instance, the stretch would need to be measured on all bolts.

It is worth noting Relaxation happens during service; as well as during installation but is a whole another discussion.