Equipment hire business HSS has bought 100% of the shareholding in the Milngavie-based company for an undisclosed sum.
Nevis Capital, formed by James and John Pirrie after their £62m sale of LCH to Speedy in 2006, confirmed it had exited but was bound by confidentiality agreements over how much of a return it had made.
Nevis bought Woodside Power Generation in November 2009, in what was reported as a multi-million pound deal, then rebranded the business as Apex.
The company has a fleet of more than 530 generators which it deploys in sectors ranging from construction and industrial to events and oil and gas. All 26 employees at Apex are staying on with HSS planning to keep the brand alive and run the business within its UK wide specialist power division.
Initial talks on the transaction began several months ago when Nevis approached private equity firm Exponent, the owner of HSS, to find out how the integration of the Abird temporary power business purchased by HSS in October 2012 were going.
Brian Aitken, one of the founding partners at Nevis Capital, said Nevis had looked at acquisitions as a way to grow Apex.
But following talks with HSS it was decided the combination of Apex with Abird would provide the opportunity to create a market leader in the UK.
Mr Aitken said: "For Nevis it gives us a lot of firepower to get new deals done, which we are very keen to do.
"It gives us a good return to go and reinvest in new deals and lets Apex grow and prosper.
"We have no doubt that (Apex) is better together as part of a national (UK) operation such as HSS where it can continue to grow and develop."
Documents available at Companies House show for the 12 months to March 2013 Apex made earnings before interest, tax, depreciation and amortisation of £1.5m and pre-tax profit of £644,000. Turnover was around £4m in the 2013 financial year.
The business has increased its generator fleet by around 75% since the 2009 takeover with revenue also believed to have grown substantially during that period.
As recently as May last year Apex confirmed it had spent around £1m acquiring a further 70 generators in response to an upturn in trade.
It also signed a deal with offshore equipment supplier Haz Hire to take its generators into the North Sea oil and gas market.
Yesterday Chris Davies, chief executive at HSS Hire, said: "The addition of Apex to the HSS Group is another solid step in the growth of our company as we continue to increase our presence in existing markets and expand into new ones throughout the UK and Ireland."
Nevis's other investments also include the temporary power supplier Dieselec Thistle, satellite builder Clyde Space engineering group The Micro Spring and Presswork Company as well as the boiler specialist James Ramsay.